The X-15 was marketed as "small", "medium", and "large", with perhaps a couple of other variations.
Small accepted 2-3" revolvers and some compact semi-autos.
Medium accepted 4" revolvers and many "service" sized semi-autos (1911, S&W 59 series, Browning Hi Power, etc).
Large accepted the 6 and 6-1/2" revolvers like the Model 29, 28, 57, 27, etc; some Colt single-actions, Ruger Blackhawk and similar.
Accommodating multiple handguns was accomplished by the use of a U-shaped spring closure at the holster opening, with the spring holding the handgun in place and allowing it to be pulled forward out of the holster by the spring opening under pressure. With this design the actual dimensions of the handgun were less critical to holster fit because the spring forced the holster closed over the handgun, rather than relying on direct fit to the holster pocket.
Overall, this is a solid and functional design that usually held up very well over extended use. A bit complicated to manufacture. Spring shapes were critical to proper functioning. To retain spring tensioning the springs must first be formed and tempered, then incorporated into the leather holster, making for some challenges in assembly and stitching. These factors made the X-15 a pretty pricey choice back in its day (roughly 1960's to 1980's).
The single weakness of the X-15 was always the elastic strap over the off-side shoulder. Worked just fine when new, but over time the elastic became worn and lost its ability to stretch and contract. I have done many conversions on the X-15's to replace the elastic straps with leather, usually with a belt tie-down point on the off-side to stabilize the harness and help support the load in use.
Another issue to watch for in used X-15's is the leather covering the spring, which can become worn through over time and expose the spring, which can scratch the handgun finish badly.