Some workshops have their platform number, not chassis number listed on the ARN lists, but alas no workshop trailers show up withthat chassis number in the listings. 2x No.5 trailers though, one built in 1963.
Interstingly,101-389 and -390 built in 1962 has no chassis nymber listed for it. Not until 101-393 does a chassis number (No. 1) appear) and they are listed as 1964 builds. So if the date is correct, potentially one of those two platform trailers is your one. Unless the platform was moved off a workshop truck that is.