I bet this is a frame built in CB factory in early 1958, shortly before Holdsworthy takeover. I'd guess lay in stock on July acquisition and subsequently painted and decaled by Holdworthy with their newly designed CB signature badge with London address on seat and head tubes. I've seen a couple other frames thought to date to 1958, with same signature badge, also with a similar stamping sequence along BB edge - different than earlier Claud numbering. Other frames have numbers 458571 and 558774. Wonder if your #358283 signifies May 1958 and 283rd build??? A bit of a grey area.
For 1959 model year, newly launched Holdsworthy Claud frames started with a simple sequential system that progressed presumably from 1 until year 1976 when numbers reached early 70,000's. #358283 as you say doesn't fit in this serial system.
See Kilgariff's site and "Frame Dating" page that links to Dave Scrimshaw's table and graphs describing Holdsworth and CB serial systems.