The M-21 is a 25-tube rocket launcher developed in 1953 during the Korean War.
Capable of direct and indirect fire, the rockets had a maximum range of 5000 yards and could be fired individually, salvos, or all fired in 2.5 seconds.

The Museum's example, made in
1954, is SN 208 of the 1,200 that were built by Rock Island Arsenal.
It shows very well and is complete with the rare-to-find optical sight and electric fire control mechanism.