Dan,
Thanks for the photo. From my notes I've seen mold cavity numbers 28, 53, and 89. The 89 is the original from my 66, now in several pieces. These things harden from heat and age and are very difficult in my experience to remove from a valve cover without breaking apart. Also, all the ones I've seen also had the "1" on the other side.
Dave
You are welcome. F.Y.I.... years ago, twenty years maybe, SFM5026 had a grommet from cavity "18" and there was not "1" on the other side. My notes have cavity numbers from that low of 18 to a high of 111. I have no doubt there was at least 111 cavities. My only question is how many molds were required to have that many cavities? I put no significance to the cavity number as Ford required many thousands of grommets for all the 260/289 Ford engines they were to be installed in. As the grommets were made all parts more than likely went into large bins for lift trucks to move away from the molding presses.
Here is an ex-1964 Fairlane cavity 28 grommet shown with the valve and elbow in excellent used condition Jim Sfetko took a picture of. The way late Cobra owners and their restorers find parts is ask for Fairlane parts. The valve shown was apparently a Cleveland engine plant only item. It is the second design used for the 1964 model year. The first design valve was used as the only known service part.
Note that this cavity 28 part does not have a "1" molded in. In my 1964 Fairlane and Cobra part picture collection about fifty percent have the extra number.
I have had mixed results heating old grommets to remove or install them. Sometimes heating seems to help and sometimes the rubber splits. I have not tried heating them in 1960s style Ford automatic transmission fluid (which will soften some types of rubber at least a little).