port one inlet, port two outlet, and some of the solenoids have flow direction marked by arrow sign.
on burkert solenoid, " P " is inlet, " A " is outlet.
you can actually use a clippard mouse solenoid, the one with the 10-32 ports, but require a couple more fittings from clippard to connect to pipe fittings, not recommend though, because it kind of weak, here are the part number:
11999, 10-32 to 10-32 hex connector, brass, really weak
15453, 10-32 to 10-32 nipple, stainless steel, stronger than the brass(but still weak, imo)
2CPF, 10-32 female port to 1/8" npt male bushing/reducer, brass
4CQF, 10-32 female port to 1/4" npt male bushing/reducer, brass
15036, same as 2CPF, stainless steel.
regular pipe dope or teflon tape are not ok to work on these fittings, better to use Loctite thread lock sealants.