Photos of your trailer and the plaque in question would help with identification.
Your serial number O-4303 is currently thought of as being a 1959
22' Falcon. It is not unusual for the model year on the title to be different. However, I don't know where the information on serial numbers came from, so it is possible that it really is a late 1958
trailer. I am not quite sure how we'd prove it one way or the other. Hopefully the former VAC Historian Fred Coldwell, will see this and comment, because he did much of the research that has resulted in our current understanding of airstream serial numbers.
As for the plaque, if it is what I think it is, they were used on caravans, but all club members could get them, even if they did not participate on a caravan, so that alone doesn't prove anything.
A 1958-59 Airstream is of the correct age to have participated on the 1959-60 Cape Town to Cairo Caravan, but #1894 was not on that caravan. Nor was that number on the 1963-64 Around the World Caravan. Those are the only two caravans that I currently have participant lists for.
In the 1960
WBCCI Membership Directory, #1894 was assigned to Don & Louise Shaffer of Port Huron, MI. They are not listed as having participated on any caravans. By 1966
, #1894 was assigned to someone in Minnesota, who may or may not have had the same trailer.
I hope this helps in some small way.
Here is O-4304 (erroneously listed as O-4303): http://www.vintageairstream.com/arch.../59Custom.html