I beleive Simson and Mauser started using aluminum bottom mags around 1930, maybe a little earlier for Simson.
The receiver has a date of manufacture- 1918. The 1920 stamp is a Weimar era property mark, not a date. I believe it is an Erfurt receiver. You mentioned it had a C/RC stamp, which stands for Revisions Commission. This stamp indicates a serviceable part that has failed inspection for non-critical tolerances or external flaws. The C/RC stamp approved the part for service.
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Mike C.
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