The Leatherwood ART II was later than Vietnam. The Leatherwood MPC was a less expensive scope that was much the same as the ART II and MPC stood for military/police/civilian. The military never used them.
The Vietnam era scope used on the XM21 was the Redfield AR-TEL scope with a specific to it mount. There are tons of threads here, just do a search for XM21 and or Redfield AR-TEL. (ps Leatherwood was instrumental in the development of the Redfield AR-TEL)

 

What kind of $$ will I be looking at for a Redfield AR-TEL scope if I can find one? Thanks

 

ART/MPC was a civilian scope. Here’s the military version. As to your question, $2k to $2500 for an AR TEL is avg price based on one or two sold this year.

https://www.m14forum.com/threads/chronological-history-of-military-m14-daytime-sniper-rifle-scopes.474454/?post_id=4236830#post-4236830

 

Hold onto your hat. If lucky, very lucky you may find one in good to good + condition in the $1800 to $2200 range. But I've seen folks pay as much as $3200 with the specific to it carry can.
I paid $1200 for my scope in very good + bordering on excellent condition but without mount, I already had the specific mount I traded for. Item I traded away I paid $95 for. This was quite a few years ago, maybe 8?? My scope can, I traded a XM16E1 bolt carrier group I paid $150 for.

 

I paid 2k for mine.

 

From what I understand they were 1st used in Grenada. I obtained one later last year and did some research. A company called Ironsighter in Tulsa is the authority on them and the only place I could find that works on them and pretty much the only source for parts. Ron Smith gave me their info.

 

Grenada was early 1980s. So that would have been the then-new ART II scopes which are not Vietnam era. They were first delivered to the Army in December 1980, and presumably installed on M21s circa 1981-82.

The AR TEL scopes were all made in 1969, and presumably all delivered in 1969-70 as well. Those were the Vietnam era scopes on XM-21s.

ART II on top, AR TEL on bottom. Different eras and different prices (AR TELs sell for about $1k more than the later ART II scopes, but the ART II scope and its mount are much better than the earlier optic)

 

The ART/MPC scope generally had a Weaver base (pre-picatinny). The scopes were Weaver, versus Redfields used to make ART -TELs and IIs. One can still find good steel 3-pt M14 mounts (Brookfield, SEI, etc.) from the 80s with weaver rails. This makes for a very sturdy platform using the MPC; and considerably cheaper than original USGI ARTs.