The prototype saddle first fabricated by Lacey & Phillips, through the coordinating efforts of Capt. George McClellan, was delivered about 20-21 December, 1856, to the War Department in Washington, DC.  Its description can be determined primarily by taking the M1857 model, and adding back all the pieces and parts that were removed from its predecessor.

The primary interest in this new pattern was that it met the basic requirements, and embraced the concepts of light weight and economy of construction, and (apparently) that it was in-line with previous cavalry board intentions as to style/form.  What Lacey & Phillips delivered was not quite that, although it was likely quite attractive by the standards of the time.  In any case, this is a list of the soon-to-be deleted pieces:

• Seat cover – a single layer leather cover, possibly with a webbing section underneath to support
• Breaststrap 
• Crupper 
• Sweat leathers
• Hoods – removed to leave the leather-covered wooden stirrups

Basically, big pieces of heavy leather were removed, immediately dropping the weight and cost.  

This revised pattern piece was approved by the Secretary, with the initial order of over 300 ‘new McClellan’s pattern horse equipments’ being ordered in mid-February, 1857.  It’s not known if the initial pattern piece was returned to Lacey & Phillips or kept at the War Department, but it disappeared long ago, perhaps recycled into one of the new 1857 saddles quickly issued in that year.