Tom,
The Winchester brass is thicker than the Starline, as you found out. I could be proven wrong, but I did quite a bit of research on the 38-55 when I got my Marlin. What appears to have happened is that Winchester uses 375 Winchester brass (which is a renamed 38-55 run at higher pressures) and simply stamps whichever "caliber" they are using that batch for on the rim. Since the 375 runs at higher pressure, their brass is a little thicker. My "research" seemed to indicate that if your bore was such that you needed a bullet of 0.380" or larger diameter, you normally had to use the Starline brass. I've stuck with Starline and have found my Marlin to be very accurate and a lot of fun to shoot.