NOTE: The comments below regarding when the the Mountain Home set (set two) were released in the linked PDF are thanks to the research by Milner Snell undertaken in 2016.
We will probably never know exactly when the Strachan & Co. tokens were struck or released, however, we can conclude with a certain confidence that
Type A (S&Co) were released c1874.
Type B (MH) were in use between 1904 and 1907
Type C ("In Goods" old set) between 1910 and 1920 and the
Type D ("In Goods" new set) issued later in the 1920s.
We know the first set (S&Co) were minted c1874 as we have Douglas Strachan's diary note confirming this fact and Standard Bank's 125th Anniverasary PDF (published in 2003) confirming the S&Co were circulated widely in the Kokstad region when they first came there in 1878 (see page two).
Thanks to Milner Snell for his input on this pivotal set - the Mountain Home (type B - S&Co MH). This research greatly increases the length of time the S&Co were in use as currency tokens throughout East Griqualand (ie the extended period without a reference to "In Goods").
Email clarification by Milner Snell regarding this short paper received by email on 11th July 2016:
You were the first person I sent the findings to. The crux of my findings is that the Mountain Home tokens came out soon after 1904. I do not imply that the 1907 letter was introducing the tokens to Thring or the region but rather it referred to tokens already in use, but perhaps it should have been made clearer. My understanding is the farm was bought or resurveyed in the 1870s and the store built later.
Hello Scott
Regards
Milner