The lock is for 28-29 rumble and i believe 28-31 trunk. The stand off and the base are plated brass clad but the handle itself looks to be stainless, maybe a transition handle from 29 to 30, 28 were t handle, 29 were plated brass clad and i believe 30-31 were stainless. my 2 cents
This is an interesting latch Tom. Compared to my "T" handled rumble, the stem is further off center Than my mid to late 28 latch. I am leaning toward the trunk side, 29 to later.
These are 1930. On the left is for a trunk on the right for a rumble. As you already learned the left lock is same for 28 & 29 both trunk and rumble. The longer shaft is required due to relocating the latch in 30. 31 latch would be same as 30.