it gets treacherous to decode the size you need.
Early frame fit pumps did not have a marked size.
A bit later there was one heat embossed in the well of the handle.
on a 56cm frame as measured center to top, 1" top tube, I use a 50 as marked by Silica. I use a steel Campagnolo pump head.
The main body is 413mm from end to end including collar and to the base where the rubber washer goes.
The Overall uncompressed rubber end face to top of the pump handle when off the bike 51.2cm
Where did the 50 arrive from?

Problem is no one seems to measure these uniformly, possibly even Silca way back.