The 261 was introduced in 1954.

A 261 with "short"/higher water pump was available in 1954 and 1955-1st series trucks.

Post the codes whose locations are shown on this page. We/you will then be able to ID what you have.

The 261 probably has the later style/composition gear on the distributor, if it is the original 261 distributor. Pre '68 Dave might comment on this?

http://chev235guy.blogspot.com/search/label/Distributor%20part%20numbers%20by%20year