Hallmarks on antique spectacles

I am starting to collect antique spectacles from early 1900's and would like to know if any expert could tell what the hallmarks on these gold filled spectacles mean. The circle with a cut in the middle that precedes the markings appears in a minority of these gold filled spectacles, sometimes the straight line in the middle of the circle is vertical, sometimes horizontal.
I read it indicates solid gold but how can that be if it is marked GF gold filled? These are American Optical spectacles from 1930's as far as I can tell. Thanks for any info or links to understand hallmarks on antique gf spectacles. Here is an image to ilustrate my question.

 

What is the mark on the far left?There are no "hallmarks" in the U.S.,it has the company trademark and a gold filled mark.

 

Yep something or other and 1/10 12k gold-filled - they're pretty classic 1940s glasses.

 

The mark on far left is the onr that might indicate part or all pieces are solid gold. I was told the bridge should be solid gold but not sure. Sometimes the line across circle is vertical too, curious to know what it means.

 

It was made sometime between 1929 to 1943.The circle mark appears to be pre 1929 but someone would need to find a chart with that info to confirm.... http://www.dickwhitney.net/AOLogos.html

 

marked gold filled....means gold filled......no solid gold of any karat there.....

 

Great info on that link, thanks!!!
Still don't know what that symbol means on this item...
From that booklet it could mean solid gold 10 karat??? I was told when that symbol appears, that part is solid gold...

 

That is interesting because that symbol that precedes the markings means 10k gold goods in that AO product book from 1911, check image, and just few spectacles from the 1930's have that mark, which means there must be a difference there...

 

It does for pre 1929 pieces,but I found no info for after 1929.

 

It is a shame the link to the 1931 AO product catalog on the frames menu is not working... there must be real treasure information there...

 

The link worked for me............ https://www.yumpu.com/en/document/r...-spectacle-frames-catalogue-optical-heritage-

 

Many Thanks! Here are 2 more images with that "extra" circle mark preceding the usual markings, the 3rd image shows the regular gold filled pieces on these AO spectacles.