Here is an accessory bottle jack for the post-war Chevrolet and GMC 1/2 ton pick up trucks.  This bottle jack has been cleaned, oiled and tested.  Testing was successfully lifting a '51 GMC 1/2 ton.   Fully closed it measures 7" and fully extended it measures 16".   Gear movement is good.  It still has its original paint.  The A stamp is the mark of Ausco Corporation of St. Joseph, MI.    The  attached drawing shows 55-66 placement under the seat.  I am not sure exactly which years this jack applies to as there is very little information out there.  Please check your application and ask any questions before purchase.