>M3A1 48568 which was found on the chassis when i re-checked. Does this make any more sense.
Yes, that definitely makes more sense than 858248. What you have is another example of a track that was originally accepted as an M3 75mm GMC, & later converted by Autocar, & accepted as a new M3A1 in April, 1945. That was too late for WW II, so I suspect that was sent to the French as MDAP in the early '50s. It could have been either a regular M3A1 or may have been converted to M16A1. France got both. Can you see any hints on it? As best I can tell, all of the M3-75s were built with rollers, & I think you said yours has one? If it was converted to M16A1, you may find the original M3-75 USA Number stamped on the front bumper. Also, you may see the original M3-75 serial number stamped above or below M3A1-48568 on the frame, but it will be cross-hatched out. (You may be able to read it, nonetheless.)
>the authorities over here need to know date of manufacture.
You could tell them Autocar, April, 1945. Despite the fact that it was actually made as M3-75 a couple years earlier, the official docs state that those conversions were accepted as new M3A1s. If it was me, I'd want to know the M3-75 info as well, if possible. If you could read the crossed out M3-75 serial number off the frame, or find the USA number stamped on the bumper or under the paint layers on the hoods, it would be possible to determine when it was accepted as M3-75.