[Arran]

 Since the date was stamped on the cabinet it's possible that Philco began model 90 cabinet production at an earlier point then chassis production in order that they be ready for the beginning of chassis production. The model 21 was already in production, or it just recently ended before this, and used basically the same cabinet as the model 70, and the model 90 is similar except larger, and it would not have taken that long for the cabinet shop to tool up to produce model 90s as opposed to the tooling and engineering needed to begin chassis production for the sets. In addition to manufacturing the cabinet parts and assembling them, they also needed to be finished which involved sanding, filling, sealing, lacquering, and rubbing out the finish, and that takes more time then chassis assembly. How far ahead of chassis production they would have been producing model 90 cabinets is another question, yours may have been one of the earliest ones off the line, that is unless they started building them in April?
Regards
Arran