That was common on vintage speakers.Some came with complete
systems.The plus most likely will be the same as the red+ on
the banana plug.The RCA plug was on both ends of the cable making it easy for the user when they got the system home .They also use them on a Home Theater In a Box nowadays sometimes.Just ignore the RCA plug.Parts Express.com has banana binding post if you end up liking them.Somebody that has some do it yourself skills might be able to put a pair of banana binding post on them permanently. They are generally 3/4"centers standard.Or just use regular speaker wire. Red=+,black=-.If you want to use the RCA for sure,the center will most likely be the red plus.A lot of them don't
make very good connections due to age,and lower quality parts.These parts may be available at Radio Shack stores too.Lesser quality.Most require soldering too.The RCA's usually take only take 18-20gauge wire I hope you get a connection that makes you happy.