I believe this was a UK modification, a bar was spot welded across the magazine catch to prevent accidental release of the magazine. Never seen this on US examples. I found this photo on the internet but also think years ago when the dewats were around in the US these M3's had Britishicon proofs and the extra bar over the magazine catch?Attachment 97188

Later on there was a protector or fence issued to help prevent the accidental release of the magazine