I have two J tanks which have round holes for the sender unit. I have an 1100R tank with an oval one which looks the same as the one pictured!!!
The grey stuff in the tank looks just like POR15. The 'matches' poking through are mig welds. The two small repairs aren't covering anything up they would have been holes from damage or corrosion.
There are numbers stamped on the tank. On the baseplate of the locator 'peg'. Haven't quite figured out how to decipher them but each tank as supplied, has a different code if you look at CMSNL!!!
Been back to work today;............ The simple answer, no there is no simple answer......... the answer i got was that; white undercoat would only be necessary if the primer were not white. If you use white primer or tint a dark primer, you should be able to paint a translucent top coat on without the need to 'back-light' it.
Hope that makes sense
AL