Generally, nothing on a 19″ Rack measures 19 inches. The term 19 inch comes from the width of the front panel of the boxes that are installed in the rack (see the image below). To determine if your rack is a 19″ rack, measure the hole to hole spacing.
Rack hole-to-hole spacing
19″ Rack = 18-5/16″ (18.312″) (465.1 mm)
23″ Rack = 22-5/16″ (22.312″) (566.7 mm)
24″ Rack = 23-5/16″ (23.312″) (592.1 mm)