The Tradex (M88) one is different lineage and would follow a different S/N system. The M61 was it's predecessor.
The M61 receiver is about 1.300" in diameter, while the M70 receiver is approximately 0.985" in diameter. The barrel profile of the mystery barreled action seems slimmer than a M70 barrel. The M61 barrel, which has a slight taper from receiver to muzzle, has a diameter of 0.965 at the end of the forestock.
Serial numbers of the M61 almost certainly did not exceed 4000, and if they did could not have been by very much. They appear to be numbered consecutively along with the M61's predecessor models, the M47/49.