Hi Mr. Okon,
I had to think about your post last night...I didn't quite understand what you meant by an "1855 Springfield Dragoon"--did you mean the Model 1855 Pistol-Carbine or perhaps, the Model 1855 Rifled Carbine--both appropriate for Dragoon use.
But then I reread your post and saw your reference to "serial number 15." I am quite sure that you are talking about a Model 1855 Pistol-Carbine as many misinterpret the number stamped on the butt cap as a "serial number." I don't want you to discover, some day, that there is one or even more Model 1855 Pistol-Carbines with the same "serial number" and think that you have a fake.
There were a little over 4,000 of these made in 1855 and 1856 (3 at Harpers Ferry, the rest at Springfield Armory). There were numbers stamped on the butt cap and on the underside of the brass yoke on the attachable shoulder stocks. These numbers range from 1-20. These are thought to be batch numbers and that the stocks and pistols were stamped together after they were fitter to each other. The thought is that these were done in groups of 20 before being sent to the military storekeeper to add to the inventory. However, no records exist to verrify this theory. The theory is put forth in Norm Flayderman's Guide to Antique American Firearms, 8th edition. I tend to agree with this theory about the numbers as I have never encountered anything different in the numbering series in my research.
So, using the number to track its history may lead to a dead end. However, by 1862, only nine regiments in the Union Army recorded having the pistol-carbines. That is a lot narrower target then if you had a Model 1861 Springfield!
Congratulations on the addition of a very interesting piece of history to your collection. I am sure the list would love to see pictures of it.