I have a set of those on my '50. They are 4 1/2", 16" 6 lug. They fit just like the 4 slots which belong on it. I have tubeless radial tires on them. The original hubcaps fit perfectly. The centers protrude further than the 4 slot rims, but i do not think that this has any bearing on backspacing, but just a stylistic difference.
I have an extra one, but it is 4".

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission