I picked the up with a stack of parts on Saturday and I'm trying to figure out who made it.
I had thought Vortz originally but having check the book it's definitely not one of his. The flanges are brazed on and the rest is very nicely gas welded.
Speedwell heads used round exhaust ports, the Speedwell exhaust manifold I used on my Frogeye with a Speedwell head had round ports. Might be worth checking the Speedwell Exhaust catalogue on Marks site. If you decide to sell it I'd be interested.
Having looked at the above images over & over again, I am now coming round to believing that it could indeed be a Speedwell one. Although the shape is considerably different to the one in the illustration, the flanges are very, very similar as is the collector box & style of bending.