I was inspired by cushy1944 (spelling?). His modelgun had nice dark furniture with a foregrip sling mount.
He brought up sling mount for his vertical fore grip. It's interesting that amongst current airsoft production, the manufacturer puts a sling mount on the stock, but gives you no provision for mounting the other end.
Hilariously, I read a redditor assert you can carry the gun with no mount on the vertical grip.
A couple of things come to mind. A combat 1928A1 is absolutely historically correct. But the implications go much further. The combinations of parts as implied in cushy1944s post is correct. Played out though it's even greater he implied.
The one issue I'll cover here is that a swivel on the fore grip is not the only option. Provision was made for the sling to be mounted to the barrel. The M1A1 was retrofited with a band due to a weakened foregrip mount, but this defect is not present in the 1928A1, meaning a mountpoint might have proven durable enough in the early configuration. I have a series of period photos showing mount locations, and i'll compile them.
Enough for one post is a picture of an MGC in my armory, with a mount still in place on the barrel. This mount seems to represent the actual use of the device:
A poster I spotted on display at one of the Thompson Associations