This is a modernised version of the vintage Astrotone/ Sam Ash Fuzz.
HERE's a Youtube DEMO.
It's similar to a Fuzz Face in that it cleans up well with your guitar volume back to a nice bright gritty rhythm tone, but turns into a thick fuzz with your guitar volume maxed.
I prefer it to a Fuzz Face with humbucker equiped guitars as it doesn't get wooly.
Pictured is one in a mini 1590A enclosure, but I can build them small, medium or jumbo!