I have one of the WW2 SMLE converted to No4 mags, I bought it at the Birmingham international some years back and it was hiding at the bottom of a box covered in cosmolineamong a number of standard No4 mags.
I just assumed it was a standard SMLE mag until I saw half the spine was removed as I cleaned it up.
It works just fine in my No4 and sits in my range bag as an interesting back up magazine.