I saw on the net several pistols with frames Ithaca FJA topped with a regular M1911A1 colt marking slide and spur hammer. Some said It's rare I do not know what's the truth.
As you said in the duplicates serial numbers sequences that could be taken for a Colt, that is the reason why we find such assembling. Some people aren't probably aware what means Initials FJA or others.