On the list, Soligor lenses with serial numbers beginning with "M" are shown as "??? # Letter code lens date code unknown"
I think I have the answer for those wanting to know the manufacturer of Soligor lenses with serial numbers beginning with "M."
I found out by accident when browsing on eBay. I saw a Soligor 35mm f/2.8 lens with an M serial number, and thought it looked familiar. Sure enough, I looked at my own Soligor 28mm f/2.8, and cosmetically, it's near identical, even down to the same font. Except, my lens has a seven-digit 974xxxx serial without an "M".
According to the mflenses.com serial list,
... it was made by Kino Precision (Later known as Kiron and the maker of Vivitar Series 1 lenses).
However, I've seen other sites that don't distinguish between the number of digits in the serial number claiming it's made by Kobori.
Such as this one:
In fact, externally, this 971xxxx 35mm f/3.5 also looks identical to my 28mm, and it's claimed to be a Kobori, too.
So, take your pick: Kino or Kobori.
I have no idea why the M was added. Perhaps to identify a particular region or distributor? Regardless, the mystery is solved. More or less. ;-)