@tiny 

I agree.

The plate looks exactly like one available from Chevs of the 40s that says it is for 1940-45 and '46 cabriolets here:

https://www.chevsofthe40s.com/detail/9873/Chevrolet_Chevrolet_Car_Body_Number_Plate_Special_Deluxe_All_4045_46_Cabriolet_Only.html

And as you point out, the rivets are a dead giveaway that at least the original plate has been removed.

That leaves the conclusion that the number stamped on the plate came from a title with a mistake printed on it.

So... the best we can surmise is that the O or 0 is supposed to be a C, D, or a G (March, April, or July).

@ole-olson I also agree that it is not a original plate because as far as I know a original plate does not have the MODEL line on it.

Also on the plates I have seen has a hyphen (-) between the plant number and the date letter. 

Is there a source from which the month the vehicle was manufactured could be determined by using the sequence number (or whatever it's called) that is shown by the last digits?

It may be hard (or impossible!) to locate, but a record must have existed that showed how many vehicles were manufactured each month.

Also, it may be possible to go back through the title records, one at a time until the error was found. You might have to find a patient Motor Vehicle Administration employee to help, but images of past titles are probably on microfiche. (I had to do that once with an ex-military motorcycle where the first civilian title showed a typo between a pencil rubbing of the  number on the bike and the title that was created at that time. The clerk at the counter had said there was nothing they could do, but her boss understood and was very helpful.)

I guess it might depend on how badly one wants to know the date of manufacture.

The numbers following the year, model and month characters are simply sequence numbers for that plant during that production year. All date information is what we've covered above. GM did not keep good historical records of specific vehicles. You might be able to get state employees to help you track it depending on what records that state kept and the willingness of the employee(s) to help you on your quest.