A company in Western NY here (Lockport,,near Buffalo) by the name of 'Sports Equipment Co.' made top tang safety conversion kits for numerous popular firearms.
Most use the same idea as shown on this 870 of a sheet metal cover plate placed just in back of the rec'vr.
The 'kit' included all the parts to do the conversion for a specific arm along w/a small instruction sheet.
Probably 1930's to 1960's
Another firm was Anderson Gun Shop in Washington State (Yakima?). His were tang safety conversions mostly for bolt action rifles and the ones I've see usually left only the safety thumb piece and a small bit of the new 'tang' it slid on showing. More of a custom look than the above .
In biz in the 1950's and 60's that I recall. Maybe before and after that too, but I don't know.
Just a couple possibilitys,,it looks like a production item to me. I'm sure others have had the same idea to market a top tang safety conversion for them.
added;
Here's a link to thread on another forum w/a picture of one of the 'Sports Equipment Co' kits to convert a Savage 99 to a tang safety. You can see the sheet metal cover w/ safety thumb piece and the related pieces that went with the conversion. All probably a bit different depending on the gun of course.
http://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/8985765/1
Couldn't find any other pics of their conversions in my search.