This is what I posted in another thread. All playfields are not created equal:

Not all the way finished, but using a rebound tester, and configuring for somewhere between hardwood and bronze, I got some interesting results. The problem is trying to get a reading through the clear, which is obviously changing the hardness of the material.
Average of 5 readings just above the valley on the shooter lane. HB is the unit of measurement, but it a complicated answer as to what the scale exactly is.
MMR: 119 HB
LOTRLE: 178 HB
TSPP: 138 HB (mini playfield was 153 HB)
TH: 163 HB
WOZ: 184 HB
I will do a GB and a MET On my way back from dinner and will update the results.
Updated list with more games:
MET: 157 HB
GB PRO: 92-104 HB (92 was at shooter lane, 104 was highest result elsewhere on the untouched portions of the playfield, 3 ghosted inserts)
NGG: 181 HB
TOM: 170 HB
AFM: 177 HB
TRON: 155 HB
GB PRE: 133 HB
ST PRE: 161 HB
Updated: With the updated results, especially with the ghosted one, it's pretty clear whatever they are doing with the wood has an effect. I checked the GB PRO all over, just to make sure I wasn't getting an erroneous reading. There was also a visible difference between the softer games and the harder games as far as playfield quality and condition. I don't really know what the fix is or what any of this really means as far as a solution, but at least their is verifiable data from a calibrated test device that we can go off. Also, we do rent this gear out, so stern is more than welcome to grab some equipment for their QC shop if so desired.
So far I was really surprised by the vast differences in hardness, as a typical steel pinball would be in the 450 HB range the way the unit is configured. Keep in mind, these were the average of 5 readings from the same spot (I did some reference checks around the playfield and all results were within 5%). I always thought my MMR had some funky looking dimples, but nothing out of hand. I do Now know why my LOTR has always looked perfect, even after hundreds of plays, and it has something to with it being "harder" would be my guess. Either way, take what you will from the data, but it's not everyday you can use a $10k hardness tester for a god damn pinball playfield (I think I crossed a line of being "too into" the hobby and need to step back. Haha)

It is, but not with a clearcoat on it.

Just to add, a rebound detector is used for hundreds of types of materials. Testing a piece of wood with clear on it is what makes tricky, but the results I got don't lie. i am sure the scale is off slightly, but results showed a variation from playfield to playfield. I know timebandit did a test with pinballs and dropping them, but it was somewhat incomplete. Wood is going to give, obviously, but unless you are testing at max force, the force may be just strong enough to create superficial dimples. Like running two cars into a brick wall at 20 mph, both may crumple the same amount since your force wasn't very high. To find out the true strength, you would need to ratchet up the force progressively to see where one car stops crumpling and the other keeps going (one car may crumple 18 inches 50 mph, while the other crumples 25 inches 50 mph).

Either way, the results I got were clear. All playfields are not created equal, and most of the old ones had a higher hardness reading. Now for the reason why, I don't have a clue. I do this for living and so am familiar with testing processes, but explanations are time consuming and expensive. However, the results are definitive that the majority of the older playfields were a higher hardness reading.

The range was from 90s to 190s I think, I'll have to take a look. Im also not sure what conversion you are doing, but if you figured some way to equate a number with a size than that's a good reference point for people. keep in mind, as I said above, I'm not sure about the scale of the hardness numbers. As there is a test for wood, and a test for soft metals, but there isn't an exact unit (that I have seen) for testing wood with a clear coat on it. The unit I used was setup for somewhere between wood and bronze, which I figured was sensitive enough to see if there is a difference. Using your formula roughly, extended out to best and worst case scenarios, there could be well over 1mm of difference, and that's assuming the scale of my readings were correct. Also, it was only a dozen pins, so the sample size wasn't really that great.

Honestly, I'm in the camp of it's not that big a deal really, but where it is a problem is the fact the playfield with the lowest hardness numbers had ghosting. Not saying it is definitive, but it definitely seems correlated. I also believe that when people say they see "cratering" vs dimpling, there is actually a difference, as I have noticed it myself. A decade ago, I never even noticed my playfield, as it looks the same as it did 10 years ago, 5 years ago and this year. I did notice it when I had two nib pins right next to each other, and one looked like hammered dogshit, and the other looked hardly noticeable unless it was under the perfect lighting. Again, to me it wasn't a big deal, but it was different enough for me notice, and I usually couldn't give a shit.

I honestly think the truth is in the middle somewhere. I think there is a difference in playfields, hence the number of people bringing up the issue and my results. On the other hand, I don't think it's a major issue, unless there is a correlation between the ghosting and the hardness, but i don't know definitively.

The reality is, if I was able to get results that different, with a quick little test with no real background on the subject, then stern should too (hell, all the manufacturers). They have a QC dept, if they would invest $5k in a tester or have someone setup a process for them, the majority of the Issues would go away. I mean, how much does a playfield replacement cost them? I have to imagine it's not cheap, especially with the amount of swaps. If they had someone checking the playfields prior to installation, with the proper inspection, then bam, the majority of the issue disappears. Not saying it's a perfect solution, but doing something is better than nothing, as from these forums alone it's clear consumer confidence in them has dropped drastically. I hope for all our sakes they get whatever the issue is figured out, because they make great pins, and it would be nice to know your NIB pin isn't a ticking time bomb.