I'd have to agree that obtaining a heritage certificate MIGHT help clear up the mystery. It would likely give the "correct" engine and body numbers for that commission number. I'd also have to agree with "martx-5" that it likely really is a 1958 model, or at leaast the commission number plate is from a 1958. A very real possibility is that, sometime in the past, someone lost the original title or registration and managed to obtain at least paperwork that could be used.
Big question: What are the various numbers on the car? commission, engine and both body numbers? and is there any obvious evidence that front-end damage might have occured and an earlier but good small-mouth apron swapped in? which bumper does it have: the earlier-style "flat-face" bar or the later curved (in cross-section, that is) style, and what do the front overriders look like? Are there pressed indentations on the inner fenders that would've been there to clear the further-set-back headlamp pods of a wide-mouth apron?
Lots of ways to go here, and many of them might lead you to a less than ideal conclusion, I'm sorry to say.