That is a 75ohm coax input. I have a first gen CCII Avanti that uses the same style antenna board. The extra balun on the screw terminals is completely superfluous.
How you use the 75ohm input on these sets is counter intuitive though...When you unscrew the F-connector the male end on the wire is your tuner input (not the female connector mounted on the board as one might expect)...You can confirm by following that wire to the tuner. Since that lead is too short to reach the back of a VCR or anything else useful that you might want to connect it to I recommend you find a double ended female f-type connector (I rip them out of old cable wall plates) such as the one below and use it to connect a longer coax to the antenna lead. You will probably have much less static with a direct 75 ohm connection instead of going through 2 baluns in series as you appear to be doing.
