Very nice gun!
Not my area of collecting, but I am sure you will get several replies shortly.
Please look at the guns at this link, this place is high retail but they have similar guns like yours. You can expect 70% to 80%
of their listed value.
I suggest posting the gun here in the for sale section. You have a real collectors item, please do your research before you sale.
http://www.simpsonltd.com
Good luck! I hope you end up keeping the gun in your family, you will be glad you did.
Vern