It should be compatible with the first generation pistols. One way to check is to see if the holster is also compatible with the SW99. If that's the case, you should be good to go.

 

I have a bunch of the 6378 holsters. This holster has a mechanism that fits snugly up against the front/bottom of the frame, in other words where the rail slot is. The Gen 1 frames are unique in this area, being thicker as well as tapered. I'm almost positive your Gen 1 pistol will not fit the 6378 holster.

I don't have any 7378 holsters, but think (this is only a guess) this holster may have the same problems as outlined above.

I also have some GLS Pro-Fit holsters. These holsters are more open in the rail area and lock in with a mechanism that clips into the trigger guard. This holster fits my PPQ's P99's and SW99's. All these frames are very similar in overall size and dimensions. Meaning the rail area is standard...unlike the Gen 1 P99 which has a unique, non-standard, proprietary rail.

If it were me, I'd have no reservations ordering a GLS Pro-Fit for a P99 Gen 1.

 

This one fits the Magnum Research MR9, the PPQ, the current fullsized and compact P99s, and thus I am fairly certain it will work for the first gen P99 also:

https://www.imidefense.com/product/polymer-retention-paddle-holster-level-for-walther-p99/

However, if you're looking for concealed carry, it stands off quite a bit from one's waist line, making it very obvious even if your shirt or whatever would cover it.

 

Do you think the Gen 1 is compatible with the Walther P99 Final Edition Dual Conceal Holster?