Less than 4,000 M24's? Where are you getting this number from? Remington made tens of thousands of M24's, literally a shitload of them. M24's are not rare in any way whatsoever, countless original M24 rifles and receivers were released for sale in the past few years.
There's a lot more differences between the M40 and M24 receivers than how they're marked. It's an apples to oranges comparison, they're completely different receivers. Also, the original M40's were built in 1966 and the first M24's were built about 1988. So, the Marines didn't decide not to change any markings on the receiver, as you suggest. The first M40 receivers pre-date the first M24 receivers by over 2 decades, the M40's came first.
You also mention the Remington 40X, which has different markings than the Remington 700. The 40X receivers were only used on a few prototype M40's, a few prototype transitional M40A1's, and possibly used on at least 1 M40A1 that was built in the mid-1990's (5 of the 40X receivers were found at RTE in 1995 and returned to Remington, they were traded for 5 Model 700 receivers).
You ask if anyone here has owned any of these rifles, I'm responding to this post because I have owned these rifles. I've owned 2 original 6 digit USMC M40's from 1966 (I sold 1 last year, I still own one), I own an original 7 digit 6257xxx M40 receiver from 1968 (probably Air Force used), I own an original USMC used prototype transitional M40 that's built on a Remington 40X receiver, I've owned 2 of the 5 original USMC Remington 40X receivers that were at RTE and returned to Remington (as I mentioned in the previous paragraph, I sold 1 stripped receiver for $8,000 years ago, and I still own the other receiver, it's the earliest known USMC PWS built M40A3, built at PWS in February 2001), I own 5 original Army used M24 receivers with a "C" serial number prefix and "US" rollmark, I've owned a Remington M24 rebuild rifle that had original Army used take-off parts with a new "RR" prefix serial number M24 receiver, and I also own a bunch of other M40xx rifles in different variations. Not to mention my extensive research on almost all US military sniper rifles from circa 1900 to current issue, and my immense collection of original USMC M40xx take-off parts. I've made tons of posts about the M40 series of sniper rifles on this forum over the years, some of those posts are stickied to the top of the Vintage Section on this forum because of the information I posted in those threads. I served in a Scout Sniper Platoon and have first-hand experience with the M40A3. I do a ton of volunteer work for the USMC Scout Sniper Association, and I'm on the board of directors at the Marine Scout Sniper Heritage Foundation. You asked if anyone in this group has any of these rifles, I'm pretty sure that I meet this qualification.
So, to answer your question, all USMC M40's are marked "Remington Model 700". They don't say "M40" on them, and they certainly don't say "M24" on them.
If your receiver has "M24" stamped on it, then it was not made for the Marines. I highly doubt that you have an original USMC M40 receiver of any type. There's only about 15 or so original circa 1966 USMC M40's in private hands, none of them are loose receivers (some were built with loose receivers that were found, but I'm unaware of any currently loose USMC M40 receivers). If an original USMC 1966 stripped receiver popped up for sale, it would easily sell for $25,000+. I'd immediately pay that price if someone offered me a legit 1966 USMC 6 digit serial number M40 receiver! Did you pay $25k+ for your receiver? Probably not. There are some other receivers out there that have some interesting history, and those are $10,000 each for a stripped receiver. The 5 USMC 40X receivers that were at RTE in 1995 are each $8,000+. Original Army M24 receivers that are "C" prefix and "US" marked will cost a collector $2,500+. RR serial number prefix are the cheapest of the M24 receivers. What did you pay for your receiver?
Almost everything you've posted in this thread has been incorrect. However, I'm willing to look at any photos you post and maybe help you figure out what you actually have. I've never seen a short action receiver that's also marked with "M24" from the factory. I have seen scum bags who mark receivers with "M40" and "M24" themselves, and try to pass the receiver off as a military receiver. Without seeing your receiver, it sounds like you have a short action Remington 700 receiver that someone marked "M24" at some point. I'd be extremely surprised if you had an actual factory Remington made short action receiver that was M24 marked. Either way, this would never have been a USMC receiver. Post some photos and we can tell you what you have.
You should also look at my post in the thread that I link to below. I posted photos and information about 2 of my original M40 receivers (made in 1966 and 1968). This is probably the first time these 2 generations of M40 receivers have ever been photographed together, and I also discovered that the "US" rollmark on the receivers is in different locations. Both of these original M40 receivers are marked "Remington Model 700".
https://www.snipershide.com/shooting/threads/new-m40-receiver-information.7164938/
