As you have found there is limited market interest in the Syracuse Arms Co. doubles.
There is only 1 now listed on Guns International
https://www.gunsinternational.com/gu...1314_p1_o6.cfm
The first step would be to ID the grade.
There is a long post by Tom Archer here, but unfortunately all the images are gone
https://www.doublegunshop.com/forums...59&type=thread
Tom had a bunch of articles in the
Double Gun Journal starting with Part 1 in Spring 2003 and every issue 2003, 2004 & 2005
Jack Maedel's articles started Spring 2005 to Part VIII Spring 2007
Most of those back issues are available for individual purchase if you'd like to research your grandfather's gun.