Cases were bought from case makers would be bought in ,in standard lengths . Therefore if a shop had one or two on stock, they would be suitable for a variety of guns and came with "spacers to allow the gun to be fitted in the case. I would assume that cases would be bought in 28" ,that could also be used for 26" gun and 30" that could also be used for a 28" gun.
It is also a mistake to assume that all guns when sold new came with a case. Webley'sfor example sold very few guns in cases unless by request of the buyer .For instance a large order for the US were cased , spacers cut to fit ,the barrel and the stock . A previous order went out in standard packing.