The "Mystery Projectile" that caused the penetration of an M-1 tanks side armor was probably a Russian RPG-7V or similar type light anti-tank rocket with a HEAT warhead. What caused the damage appeared to be an improved version with wave shaper in the HEAT warhead. The pictures (see here) of the penetration show classic HEAT warhead circular flash and light material splash (aluminum or similar from outer warhead casing) in a roughly circular area and at between 500 - 800 mm radius around the penetration hole. It also seems as if the entry hole on the skirt plate has a ragged and enlarged hole. This is consistent with wave shaped warheads that do not have optimal performance at point of impact as the jet is still being focused. In pictures 2-5 there is virtually no spalling around the inside exit hole and immediately adjacent interior equipment. Only items directly in penetration path has been punctured or splashed with molten copper (see gun guard picture). It appears as if the jet was disrupted and started deflagrating by interior components spaced effect and material compositions. The last picture shows residual heat discoloration on the switch box, which is typical of molten metal heat transfer and short-circuit effects as can be expected when you send conductive liquid or particles into an electrical box.
The damage done is similar in appearance to that done to other armored vehicles that have been penetrated by an RPG-7. In this case, the round hit one of the few areas on the side of the tank that was vulnerable to penetration by an RPG round. Russia has continually improved the RPG-7 warheads over the last 40 years, and has sold the improved warheads to any who were willing to pay for them.