Your dimmer switch only has three contacts inside it; the feed, high beam, and low beam. Stepping on it switches the feed contact back and forth between the high and low. No variation in voltage and just one beam is lit--either high or low. Both beams are NOT on when on high. The sealed beams are very low tech and impossible to hurt in testing so just flop it on the bench, hook a ground to one terminal and touch each of the others with a hot. When the ground is on the correct post both the high and low will glow bright and if it's on wrong then it grounds through the other element and one is dim. With just three it's easy to find the right combo.

Evan