Don't know why they put that slot in there??? That's for an antenna base for the BC-611 infantry radio. It uses a smaller "rubber ducky" mast base with a four foot antenna. Realistically, the main antenna in the large MB-48 mast base behind the TC had a very thick antenna staff....3/4" at the base with 3 three foot sections narrowing to 1/2 " at the tip. The Inf radio staffs were about 1/2 inch. The photo of the Weasel shows the type antenna and base. Another overexposed shot of loading ammo...the ant base is clear.