What is with this?

[Vlim]

A memo of the 16th of December, 1971 directs that the 'Mauser Parabellum' designation was to be moved to the center of the frame side, with removal of the calibre designation on the frame.

So we have:
1970-1971: 'Mauser Parabellum' on frame side + Caliber designation on frame side.
Between the 15th of september 1971 and the 26th of november, 1971 no caliber designation was present on the barrels.
1971-1972: 'Mauser Parabellum' on frame side + no caliber designation, caliber rolled on the barrel.
1973 - end: Nothing on frame side, caliber rolled on the barrel.