Hi Chris,
Your 1921 is one of approx. 10,000 1920 & 1921 maunfactured Lugers contracted by the Reichswehr military with
no serial number suffix, a or b suffix. Yours obviously is an
a suffix.
The unit mark 4./K.3.99. = 4th Company/ Kraftfahr-Abteilung 3 weapon #99. Kraftfahr-Abteilung transaltes to Driving Detachment, or what we would call a Transport Unit. The unit mark isn't rare, but not common, as fewer were issued to such units.
I would leave it as is. The value will be deminished by half if restored. Just keep it well oiled.
If you should decide to sell it. I would be interested seeing some photos. Reichswehr unit marked Lugers are my collecting interest.
The proofs should match the ones shown in this photo
Ron