Hey Tab,
Most definitely, check your commission number and the Moss catalog will tell you the original bore size. Very early TR4's had the 0.75" bore while later TR4's and TR4A's (like mine) had the 0.70" bore wheel cylinder.
My humble understanding is that, given an equal amount of pressure at the pedal, the smaller bore size wheel cylinder generates more pressure on the brake shoes.
I had to replace both wheel cylinders on my TR4A this summer. The wheel cylinders were original (Girlings) and while doing my bi-annual brake fluid bleed, I had an "Oh sh_t" moment when entire side of the wheel cylinder tore off after tightening the bleeder screw. After examining the crystilized metal in the crack, it appeared as if that crack had existed already for some time. Whew! Better to fail in the garage on jack stands than on the open road.
Anyway, not wanting to replace only the one side, I had to source two wheel cylinders quickly. My local NAPA parts store was able to get only one Raybestos "Professional Grade" wheel cylinder. I bought it and no doubt in my mind this was an OEM piece (0.70" bore and not made in China or India). It was absolutely identical in every way to the part which failed including being stamped "Girling" with the part numbers WC37355 and 64673305 just like the original. The labeling in the package also showed a Girling part no. of 63-37355 UK. The only downside is that he could only get me one and it was $68 each with tax. If you Google "Raybestos WC37355" you'll see it for about $45 and photos with Girling on the part.
So, ultimately, I returned this one and bought two pieces from TRF at $25 each. They appeared aftermarket, were 0.70" bore and carried a part no. of GI64674185. Don't know where exactly they were manufactured but, I do put a lot of faith in TRF parts.
Hope that helps.
Bob