Is there a stock attachment lug on the rear grip strap? It doesn't appear to be one from the picture.
If not, and if the ser# on the front of the frame is a 5 digit # w/o a letter suffix, the frame is from a Commercial or perhaps a Contract sale Luger.
Makes no difference really, just an observation. Mixed serial #'d pieces end up with just that,,mixed assortment of parts.
That they function and shoot well is what we are after for a 'shooter Luger'. Some of my best shooter Lugers have been mismatched #'d guns.

The double date on the recv'r ring ,,1918 is the yr of mfg of the original upper component. The 1920 marking was added when the pistol was accepted into the post WW1 German Weimar Republic arms stockpile under the Treaty of Versailles limitations quotas.

I had an Erfurt 1918/1920 at one time about the same condition and mismatched. The stock lug had been crudely ground off, or nearly so. MAny were done like that as people thought it was illegal to own the pistol w/a stock lug in place.
I finished off the crude grinding and restored the pistol, leaving only the frame ser# and Erfurt markings and proofs in place. A rust blue & straw coloring completed the job.
I gave the pistol to my brother who always wanted a Luger.
Still shoots it regularly and it functions just fine as a mismatched #'d gun,,even with his questionable quality cast bullet reloads!