Burnside Carbine 5th Model - Unit Assignment?

[vmicraig]

Would like to know if anyone can identify the assignment for this Burnside 5th Model Saddle Ring Carbine. I'll post clear images of all the makings below - the only identifiable stock marking that might nail down the assignment (beyond serial number assignment, which I don't have nor know where to look) is the very noticeable "9" carved into the stock. I assume this is not a rack number but perhaps a unit number? Any ideas on a particular unit based on the "9" or the serial would be appreciated.

 

[UCVRelics]

Here are the Springfield Research serial # spread. Its not listed. The 9 is more than likely a rack #. Tracking down a unit when its not listed is going to be a tall order.

 

[vmicraig]

Here are the Springfield Research serial # spread. Its not listed. The 9 is more than likely a rack #. Tracking down a unit when its not listed is going to be a tall order.

I didn't see a link - could you possibly tell me the serials closest to 21780 both before and after and what units they were assigned? I know it probably means little and likely has no direct bearing on where this one was ultimately sent, but at least it will satisfy my curiosity. Gracias!