The most standard size by far for the drums for this type of machine are about 4”x4” with a 19mm( 3/4”) keyed arbor.
There are other sizes that were used by some manufacturers though.
As far as the tools being “modified angle grinders”, they’re not.
The speed of an angle grinder is too high.
These machines usually run at about the speed of a mid rpm polisher, and some manufacturers like Fein, basically made adapter accessories that could be mounted on their polishers gor using the drums.
I’m not sure who invented this type of machine, which is usually referred to as a burnisher, but one of the older models sold in the USA, maybe two decades or more ago, was made by Flex Tools of Germany, and was specifically made as a burnishing machine, although with the layout of a polisher used on its side with a drum mounted.
The Flex was available in two versions, one of which had a cast aluminum guard covering the wheel, with a handle mounted to the wheel guard, and a built in dust collection port.
These machines were originally not cheap.
The Flex machine had a street price around $800.
Later, Metabo started selling a similar burnisher that was slightly more affordable at $600+-.
The wheels have always cost a fortune.
The Metabo wheels used to be the cheaper option.
The Eastwood and Harbor Freight burnishers do appear to have a mire ergonomic layout than the older burnisher design.
