IMHO that's not a dot but just grain of a dust . The second one can be noticed on the bottom of the digit "0" and the nxet one on the "3" and smaller ones all around the pic.
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Secondly the Wk.Nr. 0210141 and 0210324 were capturred by the Swiss in 1945 when these landed on their territory. As you may see these both belonged to the 0210 batch. What is more the Wk.Nr. 50216 that belonged to the Magdeburg Schule, landed in Dübendorf on the 18th April 1945. She had the white number 10 painted on the engine cowling and the fin. So the kind of marking was used at the end of the war.
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the source:
Bücker Bü 181 Bestmann, Veeday Models 1:72 von Helmut Dominik
And the one you had posted earlier in #72 ... BN+OT Wrknr 1
6170 LSK4 43-44
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