Here's an image I grabbed off eBay
showing the 1986 (C) in the selvege:
Why 1986 when the stamp was issued in 1988? Well, the stamp was a long time in development. In fact, in 1986 the USPS held an "unveiling" in Gladwyne, PA, of the proposed stamp on the 100th Anniversary of Arnold's birth. At the time, the denomination had not been determined, and the design of the stamp unveiled in 1986 was different than what was finally adopted. But they still kept the 1986 on the "photo essay" of the new design and that is the year that is shown in the selvege as actually issued. 1986, not 1988.