German bottle dug up - need help identifying it

I have dug this bottle up from quite deep in my garden. It is fully intact with liquid still inside of it! It has a pink metal screw lid that reads "Nach Grebrauch schütteln" on top which is german for "shake well before use" (I think). There are some marks to the base too. It states "20 ml". Then there is a symbol of two angled and overlapping squares or a two-ended arrow which possibly contains a "C" and a "P"/"B". Next to the symbol is the letter "M" and the number 23. Any information about this bottle and what it may contain or be for and when made etc would be very gratefully received as I can't find anything about it online. Thank you!

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