Interesting numbers, Adam! First off, that's obviously a pretty early "Federal" 1973 1500, with that low a number. It's not terribly unusual for the engine number to be relatively close to, but off several hundreds (sometimes higher but on rare occasions lower). But the "puzzle" is in the suffix letters. "UE" is correct for a "Federal" spec engine, but the additional "SS" letters after that usually indicate "Spares Stock" or similar.
I think your guess of warranty replacement is a very good one and probably correct. If a fault had been found with the engine before it left the assembly plant, I would think another "normally serialled and suffixed" engine would have been substituted...but that "SS" engine might have been installed at that time as well.
Given how expensive they've become ($50 or so right now and due to go up in February), it's risky to even suggest that a Heritage Certificate would answer the question...but it might.
I have to go now...those voices are back! /bcforum/images/%%GRAEMLIN_URL%%/smile.gif