"Dealers and pipe smokers unfamiliar with Peterson often confuse the three nickel-mount marks of Shamrock, Wolf Hound and Round Tower with precious metal assay marks. The nickel-mount marks are instead multivalent symbols chosen by Charles Peterson and Alfred Kapp to represent Ireland... The nickel-mount marks appeared on every single nickel mount ferrule (the round-topped domes that go over System and army mount Petes) and most nickel mount bands from 1891 to 1963."
Based on examples given, looks like yours would be between 1939 and 1963. You may be able to narrow it down a bit more with the examples given here (source for above also):
Hallmarks – Peterson Pipe Notes