The washer does seem to be held captive on the thread which would explain it.
I have actually taken the injectors apart. It is a simple pintle valve design. There was nothing actually wrong with them but I soaked the parts including the nozzles in caustic soda. The idea was to remove carbon deposits. I left them soaking for a few days then reassembled them.
Result ... same as before. No discernable difference.
I assume the blowbye is from the thread on the main body where it screws into the head. Not the top and bottom parts of the injector that screw together? Would a simple copper washer not do? Not tried it but I have got a couple of old static engines and the injectors just have a copper washer. Or else buy an injector on Ebay. £20 max.