Erfurt 1918/20 true double date

[DonVoigt]

Welcome.
But to get any significant info, you need to post good pictures.

If it is marked 1918/20 - and not 1920 over 1918- it is just what you say you think it is; they are not rare but uncommon.

Being a mismatch likely puts it into "interesting" shooter status.

Here is a picture of the receiver on my example.