AFAIK Selecta only came in ISO threading, so the bike should be a "standard" 68mm shell with a left hand-thread on the drive side and a right-hand thread on the non-drive side. You state that you got the left-hand side retainer of a cartridge in, which would confirm this.
If you're having trouble getting the cartridge to go in, make sure you clean (and really clean) the threads well. If there wasn't much grease on the threads to begin with, then some corrosion may have built up, which definitely makes a new cup hard to get in. A good idea would be to grease the existing Selecta "cup" and screw it all the way in and out a few times to chase the threads. If not, get hold of a dedicated bottom bracket die.