This model valve was used on 75 and 76 CB550 F and Ks.
There is a rubber, slotted, D washer that seals between the stand pipe/sock plastic part and the valve metal body. Without that, the reserve standpipe will be bypassed, essentially eliminating or at least mitigating the reserve function, since it will feed from the bottom of the tank in both on and reserve positions.
Sorry to hear they are $100, though. Sometimes a cheap bike has hidden costs.