The only obvious difference I can see is that the 40th is missing the lug for the strap (note the positions of the strap in both photos. Can anyone with more knowledge comment on this please.
The lug for the strap has nothing to do with the difference between 40th and vintage, but rather is to do with the earlier and later vintage versions. All Action Man guns with slings attached originally came with the sling represented by an elastic strap attached to the gun by tiny brass rings. In the later years as a manufacturing cost cutting measure all guns had a plastic strap with the lugs added to the moulds for the weapons to accommodate the plastic strap.
The 40th Schmeisser is a repro of the first release version with the elastic and brass rings.
I've just compared a vintage and 40th and the only difference is a tiny circular mould mark on the magazine of the vintage version. Other than that, nothing. The 40th is a cracking repro of the original.