I hadn't done a playfield swap yet , I believe the size would be #6-32. my approach would be to count the machine screws taken out of the top and add a few to drop

they should have them at Home depot , take a screw with you to verify the size

http://www.homedepot.com/p/Everbilt-6-32-Coarse-Zinc-Plated-Steel-Tee-Nut-4-per-Pack-802261/204274190

little cheaper at pinball life http://www.pinballlife.com/index.php?p=product&id=231

Do not reuse the old ones, it's not worth your time.

You can get 100 of them from Bolt Depot for the price of 6 at Home Depot:

https://www.boltdepot.com/T-Nuts_Zinc_plated_steel.aspx

Also, don't use the wood-screw-type mini posts. Drill the holes all the way through and use #10/32 mini posts instead with a nylon lock nut and washer instead of a t-nut underneath. You'll be glad you did.

http://www.pinballlife.com/index.php?p=product&id=320

Yeah, it's worth removing wood screw posts from high impact areas (direct line of flipper fire, pop pen, top arch stop....)

Just make sure there is clearance on the backside of the playfield.

Just take note, one of the holes in the upper playfield is in the wrong place
I think it is from the end of the ball guide in the shooter lane