Research the CA DMV "YEAR OF MANUFACTURE (YOM)" license plate program, and the CA "Vehicle Industry Registration Procedures Manual."
My quick read of both says yes, for a 1943 trailer, you'd need a 1941 base CA trailer plate, a 1942 metal strip, and a 1943 "V" tab. I believe all 3 are steel.
As long as the base plate is clear in the DMV system, you should be good.
A couple of tips.
I suggest making an appointment with DMV rather than just walking in and waiting. Be prepared to ask for a manager or supervisor.
As long as the plate, strip and tab are legible and not too rusty or beat up, don't "restore" them until after you complete the registration.