Hi Bill,
I remember years ago,Jersey Late Greats,after compiling a multi-year extensive/exhaustive resarch project on 58-64's, mentioned seeing this occasional,incomplete rear engine code stamping on the 61-63.This happened infrequently,but did occur as you see.The front engine pad was the primary pad during assembly so those were almost always correct and complete<vbg>.They felt it was simply occasional human error.And you are correct,the number 1 at rear of block does designate that your engine was scheduled to be used during the 1961 model year,which it was<g>.Hope this helps,Jim