How common were house rules in the wargaming community at the time? I feel that for most types of games what's in the book is considered canon and probably mandatory, and RPGs are a notable exception.
This did run counter to chit-n-hex wargames that tend to follow written rules closer, but those also weren't nearly as DIY in approach in general.
FWIW, most common household games tend to houserule. Lots of people's weekly card games have various options or houserules that are used. Almost everyone houserules Monopoly, and Risk, and a plethora of other common games.