Using Penn catalogs to date a reel is very helpful, but don't go by the illustrations in the catalogs. Penn used same illustrations for many years of catalogs, despite some cosmetic changes that should've been illustrated, especially the changes in handle arms and knobs and counterbalance.
So, back to my point. Starting in 1950 Penn reels now had visible stamped part numbers on most every part. So if your reel has part numbers, you know it's 1950 or newer. The slotted rod clamp phased out in 1954 is another telltale sign. Another is type of spool, and also type of knob and blade, and yet another is for senators with bushings and when those switched to ball bearings.
So, here we see a numbered 9/0 with slotted clamp, bushings, counterbalance handle, and 3 piece spool. Easy to say this one is 1950-53