Very good pistols, these are sometimes used in the 'Muscle Power League' (MPL) Competitions (see the General Airgun Section).

Check the condition of the breach seal, if that is not leaking then it sounds like the main piston seal (assuming the spring/cocking effort is strong).

You will see many similar, and maybe some literature/manuals on the Vintage Diana Airguns website (Network 54).

'Original' was the export brand, Diana was the domestic German brand.

The date is stamped on the left side of the cylinder, near the rear sight.

It has decent power and a strong recoil, it's more advanced sibbling the Mod 6 is recoiless (identified by pinion covers on the sides of the cylinder).

HTH