I’ve never heard of this M-274 replacement.  Looks like an interesting project.

“6×6 Economite Amphibious Prototype
US-ARMY
Built 1962
Serial #1

All Aluminum hull, amphibious all terrain vehicle.

Tested by
U. S. Army Transportation Board
&
U. S. Army Tank-Automotive Command
during 1962-63
as mentioned several times in report

Designed to replace the M274 Mule
Will carry 800 lbs at 25mph over land and 4 mph in the water.
Don’t have time due to new business venture”

View all the information on eBay