The 55 series wheels actually became available in 1968 on the 50 series. Your codes are correct for 18.4-38 rears with double bevel rims, which is another name for those 55 series wheels. They have 2 bevels , or ramps that your wheel wedge blocks can go on to give more spacing options. The older single bevel contacted the cast center and could only be swapped side to side to get a different spacing.

Your A code of 480 is the style code. I hadn't seen any for 1950-T Cockshutts, so I can add that one to my list. Is your adjustable wide front, underslung axle, or wheatland axle? That does affect the A code.

The W code is sheet metal for a 1950-T with dual air cleaners. This was most of them. Only the last run in 1969 had a single air cleaner.

The Y code is for accessories. I'm guessing that your tractor has a cab on it. I haven't figured out the codes for cabs. The other things for the Y code are loaders and fork lifts.

I've never seen a 1950-T without the by-pass filter, but someone could easily remove it. It is a bit of extra work to change it without spilling oil. Who knows why people do what they do, but it was probably removed at some point.