There are a couple confusing differences between later and earlier Line SI Bushmasters. One of those is the different front receiver/grip frame screws between very late Line SI bushmaster and earlier Bushmaster.
I think the screw style changed with the promaster since the same screw A is sometime used on promasters.
The photo above shows two Line SI Bushmasters with the different style screws.
The later Navarone Bushmaster has the later one piece aluminum screw and the front frame hole is recessed and curved for the screw. The threads are 8×32.
The bottom Terminator Bushmaster uses the more standard plastic cap 8×32 screw which sits flush against the bottom of the front of the receiver.
Hope that helps someone. Find Line SI parts here: