I do not get it, either. It's only been a year since I did this on two 261 engines but my memory is bad.
I just now checked for photos of the back of my 1954 water pump but I do not have any photos. Was there a gasket between the old-style water pump and the old-style plate on the back of that water pump? That is the gasket that you need.
You do not use the old style water-pump to head half-gasket. On the head side of the plate, you use the gasket for the newer style pump-to-head (to seal the hole in the head).
