Probably 3703414, the early 261s with the early style water pump are very desirable and getting hard to find.
Tim
That what it is, somewhere between paper & typing I messed that up. We had it running before removal, sounded nice. Need to check compression, etc., had the valve cover & rocker arms off, tops of valves & rockers arms look good.
Might try to swap in for a '58 up 261 as I don't have any 55 1st series & back rigs.