Hi Ted Eaton, and welcome to the Ford Six Forum! Clearly the seller either doesn't know the difference between a 223 Six and a 262 Six or he's not being honest, so you would need to decide that in your talking with him. Just going by a vehicle's VIN Number or a Door Warrantee Tag Number this is a very easy mistake to make on trucks that are now as old as these are. Because the engines could of been changed out many times all ready. Yes you are right to be concerned if it has the same number as the 223 that you have. Yes there is a unique Design / Casting Number to ID the 262 from a 223 Six Engine Block, it's been many years since I have seen one but I can give some good ideas as to what the Design Number looks like from my memories. Since 1961 was the first year they were made it would look much like this the only thing I can't be sure of would be the last letter or letters that it would have, C1TE-6015-A
This decodes as
C = The Decade of 1960
1 = The year when added to the above so 1961
TE = The Ford Engineering Department that ordered the part made this is the Truck Engine Department and the biggest clue that he doesn't really have a 262 Six.
6015 = Ford's Generic Part Number for an Engine Block.
A = Denote that it's the First year Part Design there can be many others used here when the parts gets running Changes, Mod's, or a slight Improvement made to it. I. E. AA, B, BB, etc.
Other posable 262 Six Design Numbers that could of been used.
C2TE-6015-A this would maybe be a 1962 number for a 262 Six
C3TE-6015-A this would be a 1963 number for a 262 Six
C4TE-6015-A this would be a 1964 number for a 262 Six
Ford would often use these numbers more than a single year so you can't tell the exact year by it, so you would also need to find the Casting Date Code that is 2 to 3 Numbers and a single Letter this tells you the exact day the part was cast at the Ford Engine Foundry. If you want more parts info on this engine family that had 215's, 223's, or the 262 Six's you can check out this link and there are many others on this site.
https://fordsix.com/threads/h-p-engine-build-223-262-l-6.73584/#post-565318
So remember if the first four numbers and letters don't have C1 to C4 & the TE than it can't be a 262 Six. Best of luck in your 262 hunt I have been looking for one of them or a late 223 too.