Many of the older firearms were designed to accept ammunition of varying power levels. Some examples would include:
-.32 Smith & Wesson, .32 S&W Long, etc.
-.38 Colt, .38 Colt Long (morphing into .38 S&W Special, .357 magnum).
-.44 S&W Russian, .44 Special (morphing into .44 magnum).
-.45 S&W Schofield, .45 Colt (and a bunch of "improved" varieties).
Lots of other examples. Also, many calibers were offered in "gallery loads", typically a simple lightweight bullet (or round ball) with little or no powder charge (relying on primer energy), for use in informal target shooting at short ranges (typically in bars and private clubs, popular with upper class folks).
Several popular cartridges were loaded to different performance levels, usually identified by labeling on the factory boxes. An example would be .44-40 (offered for Colt revolvers, Winchester or Marlin rifles). Another example would be .32 WCF (.32-20), loaded for handgun and rifles of various manufacturers. Differences in power levels, bullet weights and diameters, etc.
Probably the most enduring examples would be .22 rimfire ammo. BB Caps, CB Caps, Shorts, Longs, Long Rifles, various others. Prior to WW2 there were dozens and dozens of variants, every manufacturer offering their own proprietary designs, some of which were interchangeable but many were not.
Most firearms makers concentrated their offerings on proprietary cartridges (securing future sales of ammunition). Colt resisted chambering their revolvers for S&W or other cartridges for decades, unless required for a large contract or foreign sales. S&W co-opted the .38 Long Colt as the basis for the new .38 S&W Special, rather than starting with their own (quite similar) .38 S&W, but the target market was military contracts that already included thousands of Colt revolvers with chambers bored straight-through, capable of accepting the longer cartridges at increased power levels (although still loaded with black powder charges).
Prior to the 1960s there was no SAAMI, no standards of any kind. Brand recognition and marketing efforts were the only considerations for most firearms and ammunition.