Posted 26 January 2013 - 08:58 AM
It definitely looks like a camera filter to me - green is a very useful filter when shooting black and white. Older cameras (like TLR's and as you said movie cameras) often had relatively small lens filter fittings of around 30mm diameter, with proprietary fittings so you had to buy that camera's accessories.. They also frequently came in a leather pouch as per the Rollei example given - I would have thought that was more likely an indication of some degree of portability - I think camera is most likely, telescope second, enlarger third.
A museum or elderly camera store owner might be your best shot to help identify it, or maybe browse some antique camera forums..
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All my posts about Apollo are dedicated to the memory of MID - who knew, lived and was an integral part of, Apollo.