The earlier Senators all had bushings and 3-piece spools. The earlier side plates did have a marlin, water, and boat, but don't recall what else was different. There may have been a fishing line between the boat and the marlin. The solid bronze cast spools (1-piece) came in the early 70s if my recollection is correct. It was after that when the bushings were replaced with BB. None of the 3-piece spools ran on BB.
The red/maroon side plates were introduced with the advent of the "Special" Senators. These were higher speed reels that had the BB and solid spools.
When the Senators were originally produced, the line used was called, "Cuttyhunk" (think it was linen). It had low stretch. Then came dacron, that also had low stretch. Used with these lines, the 3-piece spools were fine. Then came mono and the spools blew up, literally. This caused Penn to switch over to the cast bronze spools. Those bronze spools were very heavy and Penn then switched to the aluminum spools (after the after-market guys started coming out with them). I think the aluminum spools came out sometime in the 80s.
The basic design has not changed. Certain components (I.E., main and pinion gears, side plates, and spools) were made with different materials. The tongue on the "clicker" was also changed from brass to SS, I believe. Other minor changes included stuff like counter balanced handle replaced by those without the counter balance, knob material and shape changed, adjustable handle length, etc. It's a testament to the guys who came up with the original design (don't think it was Penn) that it has stood the test of time these many years.
Very nice reel !! I have couple of Penn 4/0's that I think are also from the 60's but the side plate is different from your reel. Without a pic I know this may be difficult but maybe you can tell me which of our reels is older. My 4/0 left hand side plate is also black and has a marlin, water, and a boat and Senator under the bushing. The right hand side plate has Penn Senator 4/0 inside a circle. I am trying to learn more about these Senators and was interested in the different side plates and what the age of my reel might be.