I take it there are different ways of the spelling of Johnstone’s in those days?
Yes. There are still lots of variants.
Even if the person themselves spelled it the same way every time, which isn't necessarily the case, other people who wrote things down might have other ideas.
For Johns(t)on(e) and variants search using wildcards: Joh*ns*n* should pick up all but the weirdest spelling variations.
