However, it was the F2's interchangeable viewfinders (also known as "heads") that marked it as a truly professional-level SLR and was its greatest strength. By providing updated heads every few years, Nippon Kogaku was able to introduce new versions of the F2 and keep the basic body in the latest technology until production ended in 1980. Note that F2 heads were often sold separately from the body, mostly in black finish with about 10% in chrome, and it is therefore not unusual to see body/head combinations with mismatched serial numbers and/or colors.