Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

The top picture shows a "Dead Leg" oiler which is intended to "T" onto your steam line.

The lower photo shows a "Flow Through" oiler which is meant to be included as an inline part of your steam line.

In reality, they both work about the same, as the primary function is to allow condensate (water) to displace oil upwardly in each oiler, that then flows out into the steam line and with the flow of steam to the cylinder/piston provides the desired lubrication of the moving parts inside the steam environment.

As for the blank side and the fitting side on the lower oiler, I suspect the blank side is intended to simply be soldered onto one end of the steam line, while the other side provides a screw fitting so that it can allow separation like a union joint. There are plenty of the flow through types that have threaded fittings on both sides if you'd prefer!