I would still call it rare. As noted above, it seems the Fury was the most popular of the C body cars equipped with 4 speeds, but the total number is still very low compared to automatics. Less than 10% if I recall correctly.
I agree, it still is a rare car compared to the number of automatics that they sold.
It's main competition in 1965 would be the Impala SS and if you want to compare the number of SS four speeds built vs Sport Fury 4 speeds, you would say quite rare for the Fury.
If you flip it the other way and compare the Plymouth to '67 300 4 speeds, then it's a bit more common.
Bottom line, it would be cool to have, but the 4 speed doesn't add a huge price premium to the value of the car. If it's a nice, clean car, the 4 speed adds maybe 2 to 3 grand to the price. Sometimes you will see people asking $25K for an average car just because it's a 4 speed, and that's too much.
When i was growing up i remember a couple 4spd Sport Fury car's in our neighborhood. My older sister dated a guy with a 65 Impala ss 396 4 spd there was at least 2 more that his friends owned and 1 68 Impala SS 427 4spd convertible. I witnessed one of the Sport Fury's get totaled by the guy doing a burnout on main street and driver lost control and hit 2 parked cars his next car was a yellow 68 Roadrunner.
Back on November 21, 1964 at about 1:00 PM, I took delivery of my first '65 Sport Fury which was a Medium Red Metallic with matching interior 2 dr H/T with the 426S and 4-speed. At around 8:00 PM later that day, I met a '65 Goat at a stop light. At the end of the ensuing "speed contest" some three blocks later, he was in my rear view mirror and I was indicating 85 MPH. I don't know how many 383 4-speeds there were in the 1965 model year, but I understand there were 502 426's that had them. I am fortunate to have two of them, both Medium Red Metallic H/T's and no PS or AC. The first I had from '64 to '67 and I acquired the second in 2016.