Dave,
It looks like a well used and kept Parker. I have not seen a Parker butt plate that looks like the one on your Dads gun. If I had to put a value on it I would go in the $500 to $1,000 range. If it has not been messed with (ie: bores refreshed by honing) and it is still on face it would make a great shooter with appropriate loads. I would go with
www.rstshells.com 1 1/8 oz loads.
harry