Ted - 40mm. That's what the whole problem is! This was the most common filter size on the Canon rangefinder lenses, used on the 19mm, 25mm, 28/2.8, late (chrome/black) 28/35, late (chrome/black) 35/2.8, 35/1.8, 35/2.0, 50/1.9, 50/1.8, 50/2.8, late (all black) 100/3.5, and 135/4.
There are two "correct" lens hoods for the 50/1.8, one is rectangular and vented for the viewfinder, and uses a 42mm Series VI slip-on adapter. One is round, and uses either the same 42mm slip-on adapter, or a 40mm threaded adapter. The round one is less bulky, but less effective, and blocks a good corner of the viewfinder. This round hood was known as the S-42 in it's last incarnation, by which time it came with a 42mm clamp-on adapter.