My chassis is a later 1948 model (one of the last 1,500 produced). The master cylinder mounts are spaced just under 3.5" apart and the holes are un-threaded. I had to make sure I ordered the correct master.
That cap did not want to come off, which prompted the replacement of the whole master cylinder. It's the smart move because one metal line was ready to turn into rusty dust anyway. I ordered a new brake light switch (on the left) and Y-fitting (next to switch) so I'll have a complete new master cylinder to install.