Mike that is interesting and no doubt there were different practices, but it is clear from M'Harry's descriptions (1809) that his mash was not strained. He refers to the need to grease the still (and he worked with pot stills at the time) and heat the mash to ensure it would not stick to the metal.

He noted this was not a problem for a second distillation, which is self-evident of course.

Backset must have been a combination of the stillages of both distillations (where there were two, he makes it clear that doubling was not always done).