For your 40mm #Panerai watch you need a 22mm wide strap. For a 44mm Panerai you need a 24mm strap. And, for a 45 or 47mm Panerai watch you need a 26mm strap. The second number i.e. 24/22 or 24/24 or 26/26 is the width of the strap at the buckle. A 24/22mm has a slight taper and fits the Panerai OEM buckle as well as many of the TC buckles.
A 24/24 needs a 24mm buckle or, quite usually, these non-tapered straps have a sewn-in buckle.