You got me to open the Cambrai to Sinai rules for the first time in 30+ years. There are two scales you can play the rules at: 1:80 and 1:300. At 1:80 figures are individual, at 1:300 a section is two bases. Firing is done at 1:80 by picking a firing group of 1 or more figures (up to a section) and adding up combat factors. The target can be 1 or more figures. Not quite like IABSM/RF where, as you indicate, a single unit of figures fires. At 1:300 any number of bases fires at a single base only, also adding up combat factors for the firing bases. While I really only looked at the first few pages for scale, unit definitions and also the infantry firing procedure pages, I did notice the rules are often split into at 1:80 follow this process; at 1:300 follow this one instead.