According to Inliners, the block casting number comes up as a '54/'55 1rst 261. That date code is very strange, as the 261 was not offered until '54. Perhaps a replacement block cast in 1963? The '54 and '55 1rst blocks are a little different from the '55 2nd and later, mainly the water pump mounting. By your use of "late '55" in the thread title, I take it your truck is a 2nd Series? If so, seems odd that someone would put the early style block in, considering that the proper late style block would be more common.
