The rule is, to leave space sufficient to write the minimum m between the words, of the same size of that letter used in the body of your writing. [...]Note too the date, 1873 was the same year the first 1000 typewriters in the world went into production, so this wide sentence spacing can (once again) not be blamed on typewriters.
After an interrogation or exclamation point, and following the period, space enough should be left to write a double m.
But for the most part, in penmanship manuals and especially in typewriting manuals I'm finding this topic simply gets no mention at all. My belief is that wide spacing was such an ingrained habit from the time people learned to write, that it was simply automatic.
So I was happy to find this tidbit from The Phonographic Magazine, Volume 12 (1898), By Jerome Bird Howard, on pages 206-207:
I like this because the wide sentence spacing is assumed in either choice without any analysis at all. But also because it demonstrates that if anything, people felt that typewriter spacing might need to be less than traditional typeset spacing, rather than more.TYPEWRITER SPACING AFTER PUNCTUATION MARKS.Are there any stated rules for spacing after punctuation marks in typewriting? I notice that some writers put a space after a comma, and two spaces after a period, while others do not put any extra space after a comma and only one space after a period. Which is correct?
Answer.—It is our observation that in the best specimens of typewritten work there is always a space following a comma, and two spaces following a period.