Hi Ben, It's a threaded hole in the plate on the original. 4 studs is not a bad idea as I had to repair a tank that the bolt had been done up tight and made a hole in the tank because the original bolt has a domed nipple on the top of it for some reason. I have noticed that my ES2 has had the same thing happen and it weeps petrol down the bolt.

It may be a repro tank but the bottom of it looks good, or the plates have just been replaced. In fact looking at it again, I can't see why they would make the domed holes under the plates if they were going to put studs on them. I think it's a modification for the reasons I have pointed out.

Thanks for your input Horror! The studs do the job but I like the idea of getting it back to how it was…. and I want a place to put those NOS tank mounting bolts I purchased.
Regards,
Ben.