The bumper number, CS16, is likely Combat Support Company FAV16. Back in the 1980s the Infantry Divisions were organized under the H Series TO&E Table of Organization & Equipment. You would have 3 Rifle Companies then the Combat Support Company which had a Heavy Mortar Plt of 4 4.2in Mortars, am Anti-Tank Plt with M220 TOW systems (jeep or M901 ITV), and a Scout Plt. So CS16 could be either a Scout MG carrier FAV or a TOW carrier FAV.
SignalCorp stated he has seen the so-called 'black boxes" on only a few of the FAVs he has encountered. We know from documents that approximately 80 FAV were outfitted for the M220 TOW system, which required a DC charger for the batteries used in the Missile Guidance Set. Its possible the bottom "black box" not the battery balancer, may have something to do with the M220 power or the mystery box that was fittted to Company Commander FAVs that Veterans talk about.
One Veteran account on the 9th ID Facebook Group claims they had FAVs for some of the mortars "mounted on the side". Obviously you cannot fire either the 60mm M19 or M224, nor the 81mm M29A1 or later M252 "from" the side cargo baskets. "Mounted on the side" may just be where they carried the disassembled mortars for transit between Mortar Points MP. The TO&E back then were in a massive state of flux, reorganizations going on throughout the Regular Army and AR/NG force. Mechanized Infantry Companies had 3 81mm M29A1 mortar systems mounted in the M125A2 Mortar Carrier variant of the M113. Although most of the time, we humped them behind the dismounted Rifle Platoons we were tasked to support. Then in the Combat Support Company would be 4x 4.2" M30 heavy mortars in M106A2 mortar carrier variants of the M113. Light and Airborne Companies had 3 or 4 60mm Mortars depending on the time period. Then 3 or 4 81mm M29A1 or M252 depending on the time period, sometimes carried in M1025 HMMWV. The Mortar elements in the Fast Attack Bns hasn't been found in the documentation or Veteran accounts as of yet.
Carrying a 60mm M19 Mortar on a FAV makes more sense, as it can be set up and operated by just 2 soldiers, the 81mm M29A1 and M252 you need a minimum of 3 if not 4 crewmen. In the Mortar Plt assigned to Rifle Company, we practiced for what was called a "hipshot", a hasty fire mission conducted while the mortar vehicles were in transit between MP. You would stop the truck, the crew would set up the mortar while the Squad Leader began plotting the mortar location and target location then setting up the plotting board to get the data needed to drop the round on a target being called in by an FO or responsible NCO or Officer in a Rifle Platoon being supported.
I can see Attack Company FAV mounted Mortar for the sake of being capable of keeping up with the attack, then stopping to conduct fire missions, tearing down the mortar then bounding forward to keep within range of FLOT in order to maintain fire support. Back then Night Attack facilitated by NVD was still new, most of the time night operations were conducted under artificial illumination provided by the Rifle Company 60mm or 81mm Mortars and the CSC Heavy Mortars firing Illumination Rounds, "Battlefield Illumination" fire missions is what they were called. The mortars within the Bn could coordinate to fire a combination of illumination rounds to light up the battlefield then others can be designated to fire HE on Priority Targets or Targets of Opportunity to suppress or destroy the enemy.
