The primary reason those gay angle grips exist is to skirt around stupid gun laws here in the States. Congressmen are apes who try to look busy by passing laws that have almost no impact, so they pretend that the lethality of a gun is measured by how many toys someone can put on it. There are some who prefer it but I think vertical foregrips are superior.
The foregrip in that image resembles the one of a classic Thompson, but the name "M1930 Madson" doesn't bring up any relevant data (except garbage about Vanguard) and the Thompson foregrip existed well before 1930. Hilariously enough, there were a couple of automatic conversions designed for the SMLE (that's the British bolt action rifle for you noguns fags), one of them South African in origin. None of them were produced en masse, probably because it was retarded. I seem to remember a pistol grip for the SMLE existing somewhere, maybe a Forgotten Weapons vid, but I can't remember now.