The term originated from the comedy trio The Three Stooges. In 1955, Stooge Shemp Howard died suddenly of a heart attack. At the time, the Stooges still had four films left to deliver by the terms of their contract. By this point in the trio's career, budget cuts had forced them to make heavy use of stock footage. Using this footage they were able to complete the films without Shemp.
New footage was filmed of the other two Stooges (Moe Howard and Larry Fine) and edited together with the stock footage. When continuity required that Shemp appear in these new scenes, they used a stand-in, who would appear only from behind or with an object obscuring their face. Thus the term 'Fake Shemp' was born.
Seemed oddly appropriate for this cache.
High muggle area, please use stealth, wearing Groucho glasses highly recommended.