At last, something I know a little about. These are as you pointed out, opera glasses. Little stylish glasses the ladies used to take to public performances. The magnification is very low, and most of them were made in France, a very few from Germany, (Emil Busch). These were extremely popular from right after WW1 on thru the 30's and 40's, so yours was probably made in this time frame. These usually had faux mother of pearl wrappings around the bodies of the tubes or leather in various colors. It looks like the wrappings (for want of a better word) are no longer on yours and the bare metal of the tubes are revealed, and as you said are probably brass.
As to value, I have one of these with the MOP still on the tubes which I will use my daughter's camera to take a photo of. I ran mine on Ebay once, a lady in Michigan bought them for $39 but later had buyers remorse and I refunded her money. Mine were made by "ZINGRAFF" shown one one ocular lens and his address of 164 Rue de Rivoli, Paris on the other ocular lens. I will try to post a photo later today and explain some very interesting provenence on mine.