Two styles of auto-oiler circuits were used. The first one employed a steel line from the crankcase or cylinder to the oi tank. The duckbill check is in the 90* fitting.
The duckbill is an odd size that is NLA
But, all is not lost if you can find a Lombard auto oiler check fitting as they can be repaired using a Poulan or Homelite duckbill.
Repair the Lombard check by pulling out the brass sleeve.
I used a self tapping machine screw to drag the sleeve out of the bore.
The second style of tank oil tank pressurization used a fitting in the common wall between the crankcase and the oil tank.
Easier to fix but you have to remove the cover and make a new gasket.
File or grind off the rolled edge holding the trash screen and remove tiny bushing.
The duckbill is held in position by the little bushing.