Kurt woke me from my current genealogy obsession to put out my annual forum response for 2023. We don't have a record of M58, so yours is the first we've encountered. (To others that might ask, sorry, that does NOT add value to your car... LOL) My glass contacts are by now long gone (I'm so glad I did that research when I did, as at the time I could still find people at the source that either were there at the time, or had access to historical documents), so odds of finding new contacts that know the history to this level of detail are slim, but here is what I would say about your glass based on what you've presented:
1) You have no plant code on your glass - that is normally the mark of plant 8, which normally supplied only laminated glass (not this glass)
2) You have the DOT supplier code mark (not required until 1973, but LOF started using the mark as soon as DOT assigned it, in early 1969)
3) The M5x series applies to AS-2 (non-laminated) plate (not float) tempered glass used for GM on non-windshield applications.
4) For the known models of the M5 series:
a) M51 and M52 were untinted 1/4- and 3/16-inch thick respectively. The thinner was normally used for 67 vent, backlight, and rear quarter windows; the thicker for the large (and unsupported on two sides) door windows, to provide greater stiffness. M53 hasn't been observed (yet); perhaps was reserved.
b) M54 and M55 were tinted (Soft Ray) 1/4-inch and 3/16-inch respectively (see above for applications based on thickness). M56-M58 hadn't been observed until now.
At this point I frankly can't be certain what distinguished M58 from, say, M55. But I can speculate. I doubt that it was the application of the DOT15 manufacturer number that caused it to be redefined; we haven't seen this before. However, we have seen a new glass model number used for glass simply because it was produced at a different factory (see the glass article regarding windshields produced at plant 10). Plant 8 is the plant that normally did NOT use a plant code, and that plant normally supplied only laminated (windshield) glass.
LOF skipped (or reserved) M53, so I wouldn't be surprised if they skipped M56. That would leave M57 as the next model in this set to be used, and M58 after it.
Based on their typical consistency in assigning glass models, M58 might be expected to be AS2 (check), Soft-Ray (check), safety plate glass (check), and 3/16 inch thick (we don't know the thickness of your glass, but your photo is in a backlight that would normally be 3/16 inch thick).
So my speculation based on what you've told us is that perhaps M58 was simply a rare run from plant 8 of 3/16 thick Soft-Ray AS-2 plate tempered glass (non-laminated) - and if so - I would expect M57 to be the 1/4-inch thick version.
Do you see this same glass on your doors, and if so, what is the thickness? Only rarely was the thinner glass ever applied to door windows; that has been seen, but would definitely non-normative based on earlier designs.
Can you post a photo of your door glass, and measure the thickness of that glass, please?
Thanks, Rich
