Pretty sure the M40 swivels were produced at Remington.
Same swivels as 513T.
Numrich sells the rear here.....
REMINGTON 513T Rear Swivel, Blued, 1-1/4" Loop, Used Factory Original
www.gunpartscorp.com
Fronts are found by searching parts bins at gun shows, odd parts lots on Ebay or you buy a 513T and scrap the rifle after getting the front swivel.
Some have added material/rethreaded a rear to make it a front. Id be leery it would fail if your intent is to ever sling the rifle.
Wichitas were used on the A1.
They look almost identical to me but they are a little different.
If you are not a purest by the Wichitas.