As well as neat pieces of engineering the check is also a useful means of dating early reels. Below are photographs of a few with, where relevant, some indication of date first produced.
Early or 1896 check
1906, (sometimes called 1905 -11) check
The beautiful 1912 (most likely 1st used 1911)
Mark 1 check or 1917
2 versions of the Duplicated Mark 2 check
Some say 1922, some disagree
(note: "Duplicated" is assumed to mean that parts were interchangeable, not that there was a spare set within the reel)
Post war (1950's) check which introduced the ability to change from Left to Right hand wind
The 2 parts of the transverse (edge on) Golden Prince check from the 1980's
Hardy JLH from 1990's with what they called the "compensating click check"