This isnt official development, as museum is known for trying to replicate outside look of vehicles.
They could have gone to replicate this as ISU-122-1, however, none was saved to this day so even this would be fake if we talk about if it was saved or not.
Basically museum is known for trying to replicate looks of vehicles with parts that dont entirely match it.
In other words theres no prototype for ISU-125, this right here is simple placement of 2A46 into ISU-122 done by museum. It lacks ejector.
Serial number is 241-50431.