Probable late SN-800. From wichitaband @ eBay. Unusual serial number.
The most obvious difference between the SN-800 and SN-901 is that the 901 has metal thumbrests and the right hand one is adjustable. The right thumbrest on this horn is obvously not adjustable, so I feel a bit better calling this an SN-800.
As far as I can tell, there were no SN-900s or SN-990s.