Interesting that William Bunting, who would have been familiar with all the same source material as Norman Crawford, fellowshipped with like-minded Christians, and also accounts for the origins of "tight-open brethren," designates the beginning as 1825: 

https://www.brethrenarchive.org/archive/open-brethren/fellowship-questions/spiritual-balance-or-the-perils-of-unscriptural-extremes/

See page 32 and following.