The bourbon was distilled in 1955, bottled in 1960, making it either 4 or 5 years old, depending on respective distilling/bottling dates.
The standard I.W. Harper bottling was a 4yo. Is the designation "Bottled In Bond" anywhere on the bottle? Most 100-proof bourbons from that era were BIB -- which is to say, the whiskey inside the bottle was all distilled in the same season (either Spring or Fall) 1955. That designation was long considered a hallmark of quality, though it is less so today, with many bourbons/whiskeys made from batches 'married' from different-aged distillate.
If you like bourbon, enjoy it. There is only a small and well-dispersed active whiskey collectors market, and many whiskeys were produced in large number. If possible -- I.W. Harper is now mostly export-only, except in a handful of 'control' states, where substantial domestic markets still exist -- I'd pick up a modern version and use a side-by-side comparison as a sort of bourbon history lesson. They will be different.
Enjoy!