Will this knob fit Falcon Mark 5 patio gas grill

Most likely, the valve stem opening is the same size and will fit your grill. The most important thing to note is the orientation of the knob. The valve stem makes a D shape so when you look at the back of your original knob, find that shape. On the knob in question here, the flat part of the D is facing down (at 6 o'clock) when the arrow on the knob is in the "Off" position (pointing straight up, or 12 o'clock). The orientation must match your original knob for the piece to work.