I really don't know if these guys would be able to help, but for less than $1 domestic and about $2 international, it is worth a try.
I would emphasize the DE-MILITARIZED aspects of wanting a low velocity 76 mm L23A1 breechblock.
http://www.smithconsultinggroup.net/...anadianmsa.php
The second place that comes to mind is the Littlefield Collection. There were some stories this week that the new owners had a visit from the Bureau of Alcohol, Tobacco, Firearms and Explosives (BATFE) about deactivating guns on Jacques' vehicles before the auction. You might catch the The Collings Foundation in a disposing frame of mind. Lord knows what Jacques and his crew squirrelled away.
(Why the Americans gather up all the fun things in one department but they turn around and deny their taxpayers the enjoyment is beyond me. But that is another debate ... )