F1 is the 43v fuse. Check E9/E10 connections.
J2 in your pic looks correct. Unplug J1 and 3 and see if the fuse still blows. Confirm all your keys are correct and you are not plugging in a cable incorrectly.

I'd say unplug everything from the rectifier board, and then one at a time plug stuff back in until something blows.

That will at least narrow down where to start looking

Follow all of J2 wires and see if something is touching something it shouldn't be and shorting out.

Is the rectifier board wired correctly?

Load humming...is a coil locked up?

it's hard to tell from your pics.

your new rec board has 9 pins on J1 to accommodate other games that used the 9th pin, yours doesn't and plug must start from the left, connector pin 1 to header pin 1

can you post a pic of J2 connected to the header, curious if you have it upside down?

do you have keying plugs in your connectors?

I notice you have 3 different types of connector housings for each connection to rec board.

Lost World didn't come out with IDC connectors, someone has done this in the past. Would definitely swap them out for crimp type.

Did your rec board come with connector housings, pins and keying plugs?

Pretty sure Lost World came with credit knocker in Back Box, is it there, in the top left hand side, and wired up?

Maybe try unplugging everything from all boards including rectifier and then replug everything, starting with rectifier.

And see if you can get rectifier plugged back in without blowing the fuse. Maybe something else is causing this.

Sounds like a pain in the ass but welcome to troubleshooting pinball machines

The last pic shows the old heat sink bar still in place. This is likely shorting the large bridge, causing F1 to fail. Remove the bar.

that's why you don't have the 43VDC wire in your connector 13 J3 as the credit knocker isn't there, that's what the wire is for.

it may just be that the above mentioned heat sink bar is the reason for your issue, it is held in place with the original heat sink compound and will just tap off.

what is odd id J2 is for the cabinet, and F1 is for your switched illumination, and there isn't any switched illumination in the cabinet.

That ground braid needs to be secured better. It can’t be sticking up like it is . It will make contact with the wiring on the lamp panel and cause problems.

try unplugging the door and see if the fuse blows?

with only J2 connected and the door out of the circuit it only leaves the mains coming in and the flipper switch returns.

i see the white/green thin wire has a nick out of the insulation, ideally this should have some heat shrink it, but did you check all the wires that come from the transformer for breaks in the insulation?

whilst you wouldn't want to think so being a new rec board, but it is possible BR1 is shorted internally.

are you able to check with a meter?

This is what I was going to suggest. Could be something as simple a bent lead on a lamp socket shorting it out. If the fuse doesn’t blow. He will know the problem is in the coin door.

no I don't, but that is all that is left on J2 and doesn't explain the GI fuse going open.

and now you mentioned the problem existed prior to the new rec board I'm guessing to a short in the cabinet.

yes to flipper returns inside the cabinet, the actual flipper switches themselves, and yes to only connector for the door.

with the power off check the wires in the cabinet, you may need to cut a few cable ties? I'd be concentrating on following the GI wire, pin 1 on J2 from the female cabinet side back to J2. F1 is for your GI and that's the only wire (for the coin entry lamps).
Actually with J2 and the connector unplugged, use your meter (assuming you have one) and check for continuity from the J2 pin 1 plug (the probe tip should fit in from the wire side) to the other pins in the connector, you shouldn't hear a beep and/or see infinite resistance. But in your case you do want to hear a beep. If your meter beeps then you know which wires to look at. I'd assume you will get a beep and this will be the cause of F1 going open, if not I'll join you banging your head against the brick wall.

it sounds like the GI is shorting to ground.

it's unlikely but there could be solder blob that has shorted 2 wires, I say unlikely because it would have to some heavy duty blob to melt through the insulation and join 2 or more wires.

it only takes the thinnest of stray wire strands, just one to cause a head f*ck.

it might be a bit late, but ideally you'd have purchased a circuit breaker and soldered a blown fuse to it, found on ebay, https://www.ebay.com.au/itm/403405655263 that you simply press a button resetting the fuse instead of going through fuses.

If the above fails i'd remove the wires/pins from the connector except the mains wires 6 & 7 (yellow & blue) and connect the plug and see what happens. If the fuse blows then it's the board or even though you've checked and checked the wire connections to board, it still may be them or the board is shorting on the ground plane. You can remove the rectifier board from its stand offs (to eliminate a short to ground) ensuring it's sitting nicely away from anything and power it up. If you are 100% sure the board is in no way touching the ground plane then don't bother. But the aftermarket rec boards I've seen have a connection from the rec board ground to the ground plane, so if your board has this you can remove it temporarily, just for the test.

SJ23 What happened?

Kind of wondering the same.

You sure the existing boards can't be repaired?
Usually a pretty good option.