Good eye there specialk!
That butt cap is a brown Fenwick logo cap from a 70s ultra light spin rod with a brown O ring above it. The original was all scratched up and corroded steel, about the only worn part on the whole rod. The O ring is because the curing epoxy pushed the installation out about .040". Lighter and no more clicking on the gravel either. Yes I'm a&@! retentive.
Your rod is equally as beautiful as Sandman's, and really has a personality, it must be the way the decals are applied. Mine also has the tube and caps intact, although worn as par.
I've noticed that every Fenwick has it's own casting personality too. Even within models. My B 15813 has a bit more backbone overall than another E serial FF70-4 I have. I checked my casting observation with 3 ounces of pennies hung from the leaders, and the arc of both is very similar, but the later one bends about 3" farther. That's the opposite of what most members report, and I believe it depends on how the individual blank was rolled and cured, as well as uniformity of resin and finish.
Knowing (blue collar family) how manufacturing plants operated back then, no two people that handled a rod worked with the same care or methodology, hungover morning or Friday at 3:45. Shop conditions alone played a role, I know too we're lucky they kept the grime out of the finish! Quality Control was probably sandpaper and spar varnish. Sorry Jimmy Green.
We are seeing and appreciating some of the best examples of workmanship from over half a century ago.
Thanks for the sharing, Charlie