I pop ATR's on a penny and slip them behind/onto the normal infantry base to show which have an upgrade. You could just pop them directly onto the base anyway, I just like the flexibility
TBH a mortar is a mortar in this scale - unless it's a huge one versus a tiny one (2" to 4.2" sort of thing) then anything in the "normal sized-ish" category is likely usable as anything else - and as edenviews says it also depends on year and doctrine, availability etc etc and one weapon can be covered by multiple stat lines across the period!
The 88mm only has "ground stats" as within BKC's frame of reference it's not actually useful as an AA weapon, as they'd be firing "barrages" into an area and hoping the aircraft fly through them. On table they're just part of the "ground forces" and used as ATGs/light arty.