Which part may not be present in older single-action firearms?

The transfer bar is the correct answer because it is a safety feature that was not commonly found in older single-action firearms. Introduced in the 20th century, the transfer bar mechanism allows the firing pin to be activated only when the trigger is fully pulled. This design significantly enhances the safety of the firearm by preventing accidental discharges if the gun is dropped or mishandled. Many older single-action firearms, particularly those manufactured before the adoption of this safety mechanism, lack a transfer bar completely, relying instead on the traditional hammer and sear design for firing. In contrast, components like the hammer, trigger, and cylinder have been integral to firearm design for much longer and are typically found in both older and modern single-action firearms.