This might get a bit confusing and that is not my goal but I want to share my experiences and knowledge on the topic. Probably close to 30 years ago when I was restoring our 300B I sourced the rubber for both front and rear windows the Craig Karr who was local to me and later found out he was the supplier for Gary Goers.
My body shop had a glass guy come out and do the install and for the most part I never thought much about it as it all seemed okay. After a period of time I noticed that the lower strip of the passenger side of the windshield would pop off the retaining clip. I would simply lay the palm of my had on it and give it a slight rap with my other hand and it went back on. Over the years I learned to live with this and actually look for it after driving the car. For some reason it no longer has this problem and I don't know why but I'm satisfied.
The upper back stainless had become a problem as it didn't seat properly and I was worried that it might leave the car in flight while driving. It was then that I realized there was no locking groove for that piece and it simply sat in place with the ends locked into the side pieces.
So I started checking around with my longtime friend Ken Brody who is knowledgeable on these cars and he informed me that no one offers the correct rubber these days for either front or back. Companies that were checked were Steele, Metro and a couple others I came up with. I'm thinking no one will tool up for this because there is simply no volume to justify such.
What I did regarding my rear trim strip may sound weird and possibly scary but it seemed to have worked. I took a paint stir stick and cut three small pieces about 2" long and used Gorilla Super Glue and placed them in the cavity on the back side of that with one in the center and the others towards the ends. The result was a smooth surface with the roll of the strip thus providing a secure seating surface to the rubber.
Now, the real scary part begins. After letting those strips dry on the stainless I then put the same glue on the exposed surface and very carefully positioned the strip and got the ends locked in and then applied a light pressure and it did set up and hold. That seemed to have worked as I have no more problems with loose stainless and to look at it one wouldn't know. What you must really be careful of is getting the glue on the paint as I don't know how you would get it off. I did this by myself but highly recommend another set of hands in the process. The down side to this is it may be a real challenge to get the strip off if one needs to but it was the only option my feeble mind could come up with.
I'm not recommending that one does this only sharing my personal experience with the problem. One other item is that the strips between1955 and 1956 are different and I no longer have my C300 so I can't do a comparison but do remember they were different.
John Lazenby