??? The 513T swivels are easy to find. I ordered 2 from Numrich should be here soon.
No debate there Jake; the 513T's are available and easy to find and yes, the Rem.725 swivels are narrower. We also know that the correct/PC swivels for an M40 are 1-1/2 inches, which the 513T's are not (@1-1/4") and that the front post has a machine thread, which again, the 513T's are not.
What I'm presenting are the possibilities of locating parts for your M40 build, by identifying the original sources, in the Remington manufacturing stream that these parts either originated on, or were carried over into, i.e. Rem,725/740/760/742, as well as the 513T. While the Rem.513T used a swivel body, similar to the M40's, in the buttstock (wood thread,) the Rem.725 needed a machine threaded post for the forearm (an actual pairing of swivel posts.)
Like any surviving major firearms manufacturer around, after WWII, that had participated in the war effort, I'm pretty sure that Remington was uniquely equipped to take a swivel body and pin either a wood or machine threaed post to it and then cut, form and press any size swivel loop, into it, that's called for. At some point, consumer trends or inventory carrying costs dictated that any residual parts be disposed of, in bulk. The next step, over time, is usually a "Sold Out" notice at a wholesaler and/or distributor, just when we're neck deep, into a project/build.
Most of what we're looking for is in inventories, like Numrich (love those people and thankful for what they do,) or is found in someone elses unsorted 'clutter.'
Here's a photo of someone's 'clutter.' Q. Is there anything of interest in it?
There just might be, but only if you're open to looking for something (you want for your project) where you'd least expect to find it.
Just a thought...