That part is not a blow-off valve:
This site shows two p/ns for it:
http://germanparts.com/AllParts/AllPartNumbers_73.html
One of the numbers is a Saab p/n. Looking in my parts CD, it comes up as a part used on NON-turbo, 8-valve, CIS-powered Saabs in the mid-80s.
From the diagram, it looks like the valve is actuated to bypass intake air from downstream of the CIS plate to upstream of it.
If you're looking for a decnt, inexpensive bypass valve, check out the later Bosch valve for Saabs, p/n 4441895 (Bosch #0280142110). This is the only valve Saab sells for their turbo cars, including the Viggens, which make up to 20psi stock. Because Saab superceded all earlier valves, you might be able to find one in the pull-a-part for just a couple dollars...