Accel made it a long time ago. I was looking at an early 80's hotrod magazine and it was in there for sale for around 500 bucks at the time.
I showed it to my buddy Miles.
Well he said he had once found one at a yard and asked about it and everyone he talked to said they were bad news really bad news. Blowing up your motor bad news. The engine's would lean out so much they'd go bye bye.
When you let off the throttle no fuel goes in but boost does. could it be corrected with a blow off valve I don't know. I'd rather not find out if I really cared about the engine.
Make it a blow through design there is a reason why those aren't made anymore.