Frank,

The Napoleon (model 1857 light 12-pounder) used a No. 2 carriage.

According to p. 160 of "The Artillerist's Manual" (c. 1860/rev.1863) by Gen. John Gibbon:

"All field carriages in the U.S. service, and the wheels are all of the same form and height, and fit on the same axle-tree. They differ only in strength and weight. There are two numbers: No. 1., the lightest, is used for the 6-pound gun carriage, the caisson, forge, battery wagon, and limbers of all field-carriages; No. 2, the heaviest, for the 24-pound howitzer and 12-pound gun carriages."

In addition, the following web pages may prove useful to you.

http://216.239.39.104/search?q=cache:gzYo5SkW7SEJ:www.cross-sites.pwp.blueyonder.co.uk/WBTS/weapons/carriages.html+%22Napoleon%22+and+%22gun%22+and+%22Carriage%22&hl=en&ie=UTF-8

http://216.239.39.104/search?q=cache:2M_fco3rqX4J:www.pbocorp.biz/AllMetalGunCarriagesFieldno2.htm+%22Napoleon%22+and+%22gun%22+and+%22Carriage%22&hl=en&ie=UTF-8

Mike