Hard to tell how early this model is, Garrett stamped underneath each BFO a set of letters but I don't know how to read them. Sidewinder was made between 1964 to 1972, shooting in the dark here but based from the center coil design looks like pre seventies. And upper rod connection is curved not straight like the earliest models, I would guess between 1966 to 1969. Could be wrong, Monte may know as he has knowledge of the older Garrett line.
Value, Well not much. Anywhere from $60 to $100 plus depending on collector and the rarely it can fetch more, Your dad's is a common one. Replacing the batteries is easy, remove the upper and lower four screws. Carefully side out control panel, replace batteries and reinsert panel.
Keep the unit, It will bring back fond memories both of you using it from previous hunts and of your dad. The coil is more for cache hunting, but will work on larger coins if I remember. But needs to be tuned properly to get quarters and halfs. Pennies& dimes are tough to get but possible scrubbing the soil with proper tuning, you may want to search for a smaller BFO coil for coin shooting. I do have one Sidewinder model, packed away but will post a picture of one similar to yours.
Enjoy your dad's BFO, Something to cherish for sure.
Paul