Nice French Rifle

[Steppenwolf]

Nice French Rifle posted for sale on Collector's Firearms. French Percussion .54 caliber rifle by Lizeray in Paris. Barrel length is 32”. Overall length is 48”. Nice engraved lock and fittings. Sellers name on barrel on Blvd. des Italienns. Very good condition. $1,450.00
see photos Item# AL1877

What is the "knobbie" behind the trigger guard used for?