MGC 20 rnd Mag doesn't fit my MGC Chicago Thompson

Is this normal???
I just bought a MGC 20 rnd Mag (along with a horizontal grip and foresight) to find that it doesn't fit my MGC Chicago Thompson - it fits in the guide slot at the bottom ok, and then won't fit into the next part of the slot above the space between them. It seems like the next section of the mag slot is about 0.5 mm - 0.75 mm NARROWER than the bottom.
''
Any suggestions please????

(and yes, I've checked that it IS an MGC mag!)

Sorry I couldn't wait! I solved the prob, but post the results here for future reference!

On closer inspection, and knowing I have the correct parts.......I noticed a casting burr on each side of the mag slot!''
I filed them a little, and GENTLY tapped the mag home with a brass hammer and a bit of wood (to protect the metal).

After about 4 insertions and removals, it's working perfectly! Here's a few pics, posted in a hurry!

HERE'S THE PROBLEM


A CLOSE UP OF THE PUZZLE ...... WHY WON'T IT FIT??? ''


FURTHER INSPECTION PROVIDES THE SOLUTION - REMOVE THE BURRS!!! ''


ALL HAPPY NOW!! ''



Has anyone else encountered this issue???

I can't imagine how I missed this post when it was written last October

Just wanted to say the photos are particularly good, please don't delete these, they're far too good to lose and explain clearly how you solved your problem.
These will surely help others as casting burrs can cause problems galore.