I don't know what the "earlier thermostat setup" looks like or where it's mounted on the engine, so I can't say. It's possible a previous owner of the tank you got from Steven had to repair a leak &/or rusty spot, hence the brazing work? I don't know what the other tank you have is from - it's a mix of 2 styles: OTR/OTS small bottom hose port & 1300/124 top hose port. Strange!
As for the brazed-up tank, I was only letting you know that it seems to be the "correct" style tank compared to all other 1300's I've seen.
If you decide to swap tanks in your car, I'd be interested in the one with the curved small bottom hose port. I'm thinking I could probably just have the top port reshaped to the curved OTS/OTR type by a local radiator shop. I have plans for a pseudo-OTR Berlinetta type car, and this would be a nice touch to add.
-JS.