Why have an APWM at all on a suit with armored gloves? Just carry the rifle and save 5 kilos
It doesn't have an APWM, the rifle is carried in the gloves. This looks like MegaMekLab output, and that would have an equipment entry of "Anti Personnel Weapon Mount (Right arm) Squad" or "Anti Personnel Weapon Mount (Left arm) Squad" if it had an APWM. The (APM) notation on the Mauser appears to be how MML shows that the weapon has been assigned to a location.
Having multiple AP weapon mounts can be useful, and not just in ATOW games. At the TW scale, optional rules allow the stats of the actual infantry weapons to be used instead of all being treated as an assault rifle as default. You can still only fire a single AP weapon per turn, but you can chose the best weapon for the situation if you mount/carry different weapons. For example, a suit with both gloves and an APWM might have a Starfire carried in the gloves for long range shooting while a M42B is fitted to the APWM for superior short-range firepower.
In any case, there isn't enough slot space to mount an APWM given that a PAL only has 6 slots and the stealth armor and SRM1(OS) take up 4 and 2 slots respectively. Plus saving 5kg, if there was indeed 5kg to save, wouldn't really do anything for this particular design.
Aside from the anachronisms, the design is slightly incorrect. From the placement of the armor slots and the SRM name, it looks like a SRM1 was assigned without any ammo instead of a SRM1(OS), so the BV, CBill cost and armor slot locations are wrong. The right arm slot for the armor should be moved to the left arm, while the squad would cost 1790000 CBills or 59 BV (assuming a Mauser 1200 still, 1782000 CBills and same BV for a 960).
I can hardly imagine the one shot SRM 1 to be particularly outside of the Star League's capabilities. One might as well argue no non-canon units at all in that case.
Arguably true, but then cans and worms appear. Why not Recoilless Rifles, Light TAGs, Firedrakes, LMGs or HMGs? A good case can be made for all, even with the last two originally being Clantech. Where does it stop? The Star League might have had the wherewithal to do all of that if they set their mind to it, but they were only just starting to look at the Nighthawk as more than a spec ops exoskeleton armed with infantry weapons when Amaris put a halt to all the development.