Carl,
The FWB6xx used 22 mm foresight elements.
In my many years, I have never seen metallic 22mm inserts - only plastic or variable iris'.
Please have a peek here :
https://www.intershoot.co.uk/acatalo..._Elements.html
https://www./str/olympicmarksmanequipment
Have fun & a good Bank Holiday
Best regards
Russ