Helios's automatic-diaphragm M42 standard prime — the 44M, 8 blades with an M/A switch the later versions dropped.
The Helios 44M 58mm f/2 is a Soviet standard prime in M42 screw mount, introduced in 1971 as the successor to the preset Helios 44-2. It was produced into the 1980s at the Krasnogorsk Mechanical Plant, the Minsk plant and the Jupiter plant at Valdai, and shipped as the kit lens on many Zenit SLRs of the period.
It uses the same 6 elements in 4 groups on the double-Gauss Biotar pattern as the rest of the Helios 44 family, with an aperture range of f/2 to f/16 across 8 blades. Unlike the preset 44-2 it has an automatic diaphragm operated by the camera's stop-down pin, together with a manual/automatic switch on the barrel. Minimum focus is 0.5 m and the filter thread is 49mm. Most examples are single-coated.
Within the family the 44M is the version that pairs the 8-blade diaphragm with an M/A switch, and that combination is what separates it from the 44M-4 onwards, which moved to 6 blades and dropped the switch. On an adapted mirrorless body the switch closes the aperture manually without a stop-down pin, which the later versions cannot do without a spacer-type adapter.
Supply is plentiful and prices are low. Check that the manual/automatic switch actually moves the blades, since a seized stop-down mechanism is the most common fault on this version, and inspect for haze and cleaning marks on the soft front coating. Buyers wanting a preset ring should look at the 44-2; those wanting multicoating should look at the 44M-4 or the 44-3.