The Colt 607 was the first attempt at making a submachine gun version of the M16 rifle. Few were made and all were hand assembled as prototypes for testing in the Colt Custom Shop.
Since there were no carbine length handguards at the time, Colt simply cut the rear off a set of M16 Rifle handguards and fabricated this metal part to support the rear of the handguards in conjunction with the barrel nut assembly. This part simply fits in front of your existing barrel nut assembly (with slip or delta ring installed) and the handguards will then have to be cut/fit to the space.
If you would like the simpler "drop-in" solution that does not require cutting original handguards then (Here) is a link to the Polymer 607 handguard page.
Made in the U.S.A.