For your Magpul grip with the gap, they have this. * NOTE: This TYPE 1 Kit fits all AR15/M4's as well as many 7.62x51 variants such as the Aero Precision M5 308, CMMG MK3, CORE Rifle Systems CORE30, HK 417/MR762A1, LWRCI REPR, Mega Arms MATEN, POF P-308, Ruger SR-762, and SIG716.
The TYPE 2 Kit uses taller backstraps for optimized fit with certain 7.62x51 receivers such as the Armalite AR-10A/B/T, DPMS LR308, KAC SR25, LaRue OBR, LMT .308 MWS, Palmetto PA10, and S&W M&P10.
In general, the TYPE 1 Kit will fit most all 7.62x51 AR lower receivers but there may or may not be a gap between the backstrap's 'beavertail' and the receiver. This gap is purely aesthetic and will not affect function.
So you would need a type 2 kit like this one. https://www.magpul.com/products/miad-gen-1-1-grip-kit-type-2
I got one on my DPMS LR10 and it fits good with no gap.
Thank you ! I also discovered this later on in the thread. Magpul is right on top of the AR market.