A quick look at my sources shows both PP and PPK with the bottom magazine release. Most are early, but there appear to be some in no special number range. Some are .380, some alloy frame. Perhaps it was simply an option that Walther offered and a batch was made up from time to time, as most factories do with optional stuff.
PPs and PPKs made for the Polizeidirektion Munchen (Munich Police Department) and marked PDM have the bottom release, presumably per the police request, but those are in 7.65mm (.32ACP).
As to value, your gun is worth a about double the normal 7.65 PP both because it is 9mm Kurz (.380) and because of the bottom release.
If in good condition, I would not hesitate to shoot it with standard commercial .380 ball ammo, but avoid the "hot" hollowpoint ammo. I doubt a moderate amount of shooting will decrease its value, since it is not 100 percent to begin with.