Thank you for that information.
My research found the following auction results for oil on board field paintings about the same sizes yours.
15 x 17.2" - Santa Fe Auction - 11.16.13 - est $2000-3000. Sold $3000.
11.8 x 16" - John Moran Auctioneers - 10.18.11 - est $1000-1500. Sold $4594.
12 x 16" - Altermann Galleries - 4.1.11 - est $2000-3000. $2640.
Taking these similar works of similar suze, my estimate of auction sale valuefor your Swinnerton painting would be $3500-5000..
A retail art gallery would charge at least 20% more.
I hope I have helped you.
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