Just got a response from Ithaca.
Your Ithaca double barrel side by side shotgun is a Flues Model, which was built in 1913.
Here is a little history on the Flues Model:
The Flues Model
The Flues Model was introduced in 1906 and remained in production through 1926; it enjoyed the longest production life of any Ithaca double gun. The Flues was built on a three piece lock mechanism designed and patented by Emil Flues. The lock mechanism was comprised of a sear, a hammer with integral cocking arm, and a coil spring. The frame of the Flues model was stronger than that of its predecessors, because the simpler lock mechanism required less metal to be milled from it; making reinforcement around the water table and standing breech no longer necessary.
The Flues Model was offered in 10, 12, 16, 20 and 28 gauges; and in several different grades beginning with the Field grade and ending with the Sousa Special grade.
Approximately 223,000 guns were produced between1908 and the end of 1925.
A Field Grade Flues Model currently in excellent condition is booked price to be worth an estimated $1000; the Sousa Special in excellent condition is listed to be worth and estimated $15000.
Thank you for contacting
Ithaca Gun Company
USA All the Way!!