Gary R. Sullivan Antique Clock lecture at Christie's Auction House 

Most Tall Case Clocks from the 16th (and some earlier) century through the 19th century were made and sold with only the two front leg and/or feet. You will often find added rear legs, not replacement legs. The original design intent was for the clock to be leaned back against a wall. Later design changes included all four legs as flooring became more stable and plumb to walls. Stone flooring was often uneven or unstable. Wall structures improved with more plumb or 90° intersections with floors occurring From the mid 17th century on forward, but these improvements were a luxury. As free standing Tall Case clocks became more popular, the base plinth and footprint became deeper and wider as well.

Wonderful, congratulations! :)