Brian,
Your “period charming” building looks like it was built to house a decent 7mm layout!
The slates are interesting, as you say, no “slate and a half” at the end, however, it looks like the last slate of the “whole slate” row has been cut back a little too. It seems narrower than its neighbour, and the half slate on the second row appears to be more like two thirds of the ones above and below it.
I know of an expression “to come to bricks”, meaning a portion of wall between two edges had been adjusted in length to minimise cutting bricks, presumably an integer multiple of a whole brick, with appropriate allowance for the mortar. It would appear that the building was not sized as a multiple of the slates’ width.
Prototype for everything...
Best
Simon