Here’s a followup question on guest plates that merits a response:

Would you happen to know why the Colorado DMV would issue a “guest” plate? The other types make sense. But a “guest” plate implies someone from out of state. I don’t know. I still can’t figure it out.

Tom’s response:

If you move to a different state, you are obligated to register your car with plates from that state within a certain period of time.  This has not changed since the beginning motor vehicle licensing.  What has changed is the strictness of its enforcement.

In the earlier days, the authorities were more vigorous in enforcing this requirement.  If you were going to be in the state for an extended time for work or visitation or whatever, you would get a guest or visitor plate.  These were placed on the vehicle along with the valid out of state plate.  This would exempt you from having to replace your old plates and also keep the authorities from hounding you about your out of state plates.  Most  states, if not all, had some sort of guest or visitor plate or other registration device in the early days.