Alan,
It's amazing how things work out, as we finalised Carrington Rally only two weeks ago an exhibitor from the North said, "I will bring you a mill made in Lincolnshire for your feature of Lincolnshire built next year", the next day he sent me the attached two photo's, it's a Jeffrey and Blackstone of Stamford, first time I had ever heard of this name. Then this weekend another turns up at Newby Hall, which is a much larger version of the one pictured here.
I have just researched some notes Michael Key wrote on the history of Blackstone in Stamford a few years ago, he mentions George Jeffery ran his own business from January 1876 to June 1877, he then entered a partnership with Edward Christopher Blackstone and the Company Title was changed to Jeffery and Blackstone.
George Jeffery retired early at 44 years of age in 1882 and the name continued for a further six years.
So I presume all implements we know as Blackstone were produced under the J & B name during this timeframe.
As you say all very interesting early British agricultural implement history.
Malcolm.