This is a Sterling with a semi pneumatic Auto-Rocker kit on it that was made by Sterling. The Switch is built into the frame vertically.
I didn’t think I’d ever come across one of these kits but sure enough one popped up for sale in the UK.
Here is another Sterling Auto-Rocker kit I’ve seen which Renick Miller and Bad Boyz Toyz posted a few months back:
I thought these Auto-Rocker kits were made in 95-96, around the same time or slightly before the introduction of the original Sovereign but the page on warpig (linked below) is dated 1-1-94, much earlier then I imagined.
The date of 1994 would actually make sense though since I think this is the design that Glenn Palmer consulted on (vertical switch) and make sense in the timeline of the Blazer’s production.
Edit: Was wondering why my bolt wasn’t closed and realized I didn’t add a hammer and main spring back into it when I reassembled.