Photos by Alan W. I have not seen this style grenade launcher before. It is the Colt XM148. It was the original 40mm grenade launcher for the Colt XM16. However due to the trigger design and the complexity of small parts, easily lost in the field, the XM148 was only fielded for a year in Vietnam. It was soon replaced with the M203.
You can see the extended trigger next to the mag well below.