Although the jury is still out on this one , I'd say it resembles (strongly) some I've seen over in the European corner of the world , with manufacture dates in the 1960's-1970's .... at 'boot sales' & swapmeets .
As others mentioned about glass/frames & paintings .......
It's never a good idea to have actual physical contact between the surfaces .
The only exception to this (that I know of) is when the artist/manufacturer intended the glass as an element of the piece itself .
In the not-too-distant past , many items were framed without mattings and came in contact with condensation , molds & other icky stuff from the glass that was supposed to 'protect' them . The glass would often have some transfer of material to it's surface , especially if the framed unit was moved around a bit (road vibrations & so on) .
Many pieces of art today are fully framed and matted , including glass and poly-plastics on their fronts .
The old 'standards' of 'what you do with what' have changed a bit in the U.S. , with designers & environmental/preservation-minded folk .
It was cheap and quick to just stick a painting in a frame , seal the back & out the door .... this was very common for folks who 'did' their own framing too .
One cool part of this old method was that , with evidence of paint transfer to face-glass , sometimes one could authenticate the frame elements as being the original ones (in certain situations) .
