This 50mm Minolta MC ROKKOR-PF f1:1.7 (SN 2181401) was manufactured in April of 1973 and was a regular standard lens with very good optics.
Second Letter (no. of elements): C=3; D=4; E=5; F=6; G=7; H=8; I=9; J=10; K=11; L=12.
Therefore, for this lens designation (PF) means 5-groups with 6-elements.
The coating materials on these early MC (meter coupled) lenses comprised magnesium fluoride, plus “other ingredients”, and the actual coatings were double coated in a process Minolta called “Achromatic Coating”, giving superior colour rendition and light transmission when compared to competing manufacturers.
Another term commonly seen in relation to Minolta lenses is the term “Rokkor”. Like many other manufacturers, Minolta employed a separate name for its lenses, in this case the name “Rokkor” was chosen by Minolta founder, Kazuo Tashima, as a reference to Mount Rokko in Japan, which is adjacent to the Minolta Factory
(Above information Reference site: http://www.rokkorfiles.com/Lens%20History.html)
Links for additional information on this Rokkor lens and Minolta lenses in general - see: