I decided to replace my badly corroded OEM fuel tank skid plate on my 2004 V8. I did not want to go aftermarket because my needs are light-duty and I have only ARB skids in the front (without the transmission skid, which does not fit the early V8s without rerouting the low-hanging exhaust crossover pipe).
Presumably 5th-gen parts in good condition should be much easier to find than 4th-gen ones. A fellow 4Runner enthusiast in my neighborhood gave me a good deal on the OEM fuel tank skid plate off his 2018, and I picked it up yesterday after spending a few hours comparing pictures on eBay. The parts look exactly the same, but the part numbers (and prices) are different:
Fuel tank shield aka
PROTECTOR SUB-ASSEMBLY, FUEL TANK, NO. 1
77606-35060 $730 J120/GX470/N210/N215 (4th gen)
77606-60190 $360 J150/GX460/N280/N285 (5th gen) full-length
77606-60200 $250 J150/GX460/N280/N285 (5th gen) partial/front of tank only
When I went to see my local mechanic this morning, we put the two skids side by side, and it turns out that the second-longest support leg on the 5th-gen part is 1.5" longer. That is the only difference. So he sent me to another local shop where they cut and re-welded the leg to the required length, a 45-minute job. Now it's installed and I am relieved.
The aftermarket fabricators get away with using the same skid plate for both platforms because the support brackets are separate parts!
I hope this is useful as a reference to anyone in the community who's in the same situation. Kind of funny that basically the same part with a tiny bit more metal is half the price. On eBay, they all cost about the same, around $300, almost the same as the new 5th-gen one.