appearently it came off a '96 WRX see completed listing,
Subaru Impreza 3 port boost solenoid on eBay (end time 28-Mar-10 21:45:15 BST)
if restrictors are available from Subaru I may as well get one from them.
in the mean time I've 'repaired' the old one, drilled out the broken hose tail and glued in a piece of 8mm plastic pipe Araldite should seal it in the short term, it's not a proper solution but at least I'll be able to refit it along with a 'T' piece and rstrictor and see if the system is actually working.

 

Yes, that's a genuine OEM Subaru 3-port in that link (it's the whole caboodle: bracketry, BCS itself, MAP sensor and pressure exchange solenoid).