Magazines

Do No1mk3 and No4mk1 magazines interchange? I picked up a what I believe to be a No1 mag and it fit into a No4.

some will, some won't, some do, some don't, some might.
I have a No1 that some no 4 mags won't fit, too tight at the front of the mag well. tiny bit of judicious filing with a needle file in the front corners relieved that problem. three or four strokes tops. some people experience feeding problems, mags falling out, i myself have not.
all my No 1 mags fit both the No 4 and No 5

#1 mags were made to fit #4 as a stopgap after a time in WW2. If you remove the bottom half of the rear spine and fit the mag catch at the back as you would a new mag it should be fine. A loose mag might go in fine but feed might be a problem.