Colson, in the mid-1933 year, joined the 26" clincher bandwagon, but continued to make the 28' wheeled cycles as well during this year. I wonder when they stopped making the 28" frames?
@Phattiremike 's 1933 bike above is a 28-er and my 1933 bike is a 26-er, and the serial numbers are very close. When measuring the frame specs, we discovered that the chain stay length and fork length are longer on the 28" frames. I have posted these measurements in the Colson serial number thread.
Other makers of motorbike straight-bar frames through the late 30's(38?) like Snyder and Elgin, were also making these frames that ran both
26 or 28-er wheels, but they used one frame for both wheel sizes and simply provided a fender spacer to use if you put 26" wheels on it. Not Colson though, they made two different spec'd frame sizes specific to wheel size. What do your chain-stay and fork lengths measure? (see Colson serial number thread for measurement comparison.) This will tell you whether your frame is 26 or 28..