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Hi, 

not much to be found online about these. This is the source with the most information that I am aware of, but I assume that you will also have found it already:

https://prestashop.hydrografie.synology.me/instrument-acc/462-davis-prismlevel.html

An old post in - IIRC - a newsgroup stated that proper sextant prism levels were (at that time) 59 USD and the toy Davis ones were a lot cheaper. If you have access to worthpoint (I don't) you may get a few pointers to selling prices. And one guy on Instagram or so boasted that he had acquired a bunch of sextant stuff and is auctioning it off piece by piece and received 100 USD for a Davis prism level.

I would avidly deny being a collector should ever somebody ask, but I will happily admit to being an idiot. So I would be willing to buy for 20 Euros/25 USD plus postage to Germany (if that can be done in an affordable manner). Ideally that would be for both of them. For comparison - I have recently bought a Cassens & Plath Horizon Ultra sextant (which comes factory equipped with a full size prism level) in near mint condition (sans box and certificate) for 40 Euros plus 7 Euros Postage. No secret sources, it was advertised for that price on Ebay.

Greetings from Berlin

Ralf C. Kohlrausch