3 speed with overdrive info please!

[Hawks]

When i bought my 60 c20 it has an 3 speed with overdrive. I have never really seen 3 speeds with overdrive much. I have got it on the road a few months ago and have been driving it every day. I dont use the overdrive much. Im not exactly shure how it works. I have done a little research on it. i know it has the four bolt side cover so it should be pre 1966 right? With the non synchronized first. And im not shure what the box with the fuze is on the firewall or the button on the floor. i was thinking about just putting in a four speed. and was just wondering what this set up is worth also? Thanks for any help