I'm pretty late to this thread. That Grendel was my fifth handgun (third centerfire.) I bought it after a representative from Grendel marketed it to our department as a potential backup/off-duty option; about a dozen of us got in on the deal that was offered. This was in 1988, I think.
The "package" included the gun, a tool that served both for takedown and for clip alignment, three surplus M16 stripper clips (the .380 rim fits these), a grip extender that screwed on if desired, and a zippered case that held the gun only if the grip extender was not in place.
The P10 was a neat idea, meant to give those types that carried a J-frame without a reload an option to have a lighter, flatter gun that doubled (or more) the capacity, fired as simply as a J-frame, and didn't have a magazine release or safety lever, either of which could be inadvertently actuated. The gun didn't sell well largely due to the small size of the company and its lack of marketing and production abilities, and to the narrow niche at which it was aimed. It's DNA continues in the Kel-Tec P11 and P3-AT.
I pocket-carried mine sometimes, without a holster (pocket holsters weren't really a thing back then), but never carried it on duty due to lack of a fitting ankle rig. As a lightweight blowback pistol, it was rude, but not any more than the PPK/S I got later when I switched to an agency that mandated off-duty carry, but had a very short list of approved models.
I still own the gun and its accessories. I also still own my "first centerfire handgun", which is the query of this thread. It's a Taurus Model 66 in .357 Magnum, purchased in 1987. I actually did carry that one on duty for a little while, though loaded with the department's .38 +P ammunition.