The 66th Infantry wasn't organized until 1932.
Its antecedents were tank units in WW I.
There may have been a regiment from the RI National Guard that was designated the 66th Infantry prior to 1916 and then got higher number after that year.
Elements of New England National Guard units were organized in the 26th Infantry Division.
What else do you know about your g-pa's service?
To Join the Company of Military Historians click here