M109 made to look like a ZSU-23-4 Shilka AAA. Personally I don't think it's right to use a rear-turreted vehicle to simulate a mid-turreted vehicle but that's just me.
You got it.
A possible reason for the selection of the M109 chassis would be that it has room for the electronics necessary to support the function of the radar used to simulate the "Gun Dish" - could even be an original Russian radar there. Something the M551 based ZSU-23-4 VISMOD lacked.