Well in cutting out windows in sixties cars I remembered it as being a very rubbery substance with a string thru the middle. Apparently it came on big rolls and they pulled it off the roll and put it on the glass before setting it at the factory. That apparently was a "damn" so the adhesive, a urethane "Like" substance that held the window in wouldn't ooze out into the passenger compartment when the glass was pressed in.
Here is what was posted on how it was done at the factory on a thread about window setting at the Team Camaro site. And can be found there...Setting glass - Butyl or Urethane? ... AAARG!!! - Chevelle Tech
"GM never used butyl tape or anything like it to install windshields or backlites. Period.
The body opening flange was coated with black silane primer using a flow-brush 30 minutes prior to installing the glass, and all the reveal molding clips were installed to the Body Shop-installed weld studs.
The glass was placed on a powered rotating fixture, inside edge up, retained by suction cups.
The glass periphery was cleaned using a solvent, dried, and a different silane primer was applied.
A fabric-reinforced self-adhesive foam rubber "dam" was extruded and applied to the glass just inboard of where the Thiokol adhesive would later be applied; the purpose of the "dam" was to contain any Thiokol "squeeze-out" so it wouldn't show from the inside past the edges of the interior garnish moldings after the glass was installed to the body.
After dam application, pumpable Thiokol adhesive (with a pyramid-shaped cross-section created by the dispensing tip) was applied adjacent to the dam, all the way around the glass.
Lower glass supports were fixture-located and screwed to the cowl, and rubber spacer blocks were glued to the opening flange. The spacer blocks set the height of the glass relative to the body to ensure a good fit of the reveal moldings.
The prepared windshield was removed from the rotating fixture, installed in the body opening, and pressed down against the rubber spacer blocks; wooden tonque depressors were used to "paddle" the Thiokol around the edges at a 45* angle from the top edge of the glass to the bottom edge of the body opening to ensure a continuous seal all the way around the glass.
Reveal moldings were installed.
That was the plant production process, and explains what is sometimes described as a "rope" being found when an original windshield is removed - the "rope" is the fabric-reinforced rubber "dam" that was applied prior to applying the Thiokol adhesive.
If you'll look in Section 4 ("Fixed Glass") in the Fisher Body Service Manual, you'll see exactly the same process for windshield replacement, except the "dam" wasn't included with the Service kit - masking tape was used instead to control "squeeze-out".
The Thiokol installation passed the Federal MVSS windshield retention tests, and contributed substantially to the body's torsional stiffness; it also helped the structure pass the roof crush test. It's illegal to replace a windshield with butyl tape, although some glass shops do it as a less-expensive short-cut method instead of doing the job right."
As I present in that threat at Team Camaro, these cars aren't unibodies and the glass is doing nothing but keeping water and debris out of your car.
In a collision while driving a 1968 Camaro if your windows being set by urethane were to make a difference over butyl you are in BIG trouble and that glass attachment is the least of your worries. That being said, why not do it in urethane so there is NOTHING to think about. When I was setting windows on a regular basis on butyl, I was doing it without any training or understanding of what I was doing and I feel bad about that now.
It's like the old argument that you will get from people on the forums "I do it and it works", we all have heard that over and over. Well I set windows on lacquer primed, lacquer painted pinch welds with Butyl tape over and over, and they didn't leak. That didn't make it right! That was HORRIBLY wrong, now with a little education (that damn book learning) I have learned (and it makes sense) that was miles from proper. So I am building a car, I am sand blasting window pinch welds cleaning it all out. What is the difference from that point to do it right, epoxy primer over bare metal, then protect that from paint so you can set the glass on urethane over that epoxy primer, OR you paint the pinch weld and use butyl tape? The difference in how well that glass in mounted is HUGE, the difference in work or money or time doing it is next to nothing, so why not do it right?
Like I have said my Gran Sport has a windshield set on butyl tape, over epoxy primer. I feel, it's an old car I am screwed in a bad accident anyway, that is good enough for me. But it would have been just as easy (realistically) to install it with urethane. The urethane application learning curve is VERY steep though. Butyl tape is a sure thing home hobbiest level product. Applying urethane is NOT, yes it can be done but holy cow can it be screwed up too!
So a decision has to be made by anyone setting their glass, make it and live with it.