A model 1873 could be a carbine, rifle, or musket. Easy to distinquish these by the barrel length, 20, 24, or 30 inches. There were three models of each. First models will have a serial number under 30,000, second models from 31,000 to 90,000, and third models over 90,000. These ranges are approximate, so if your serial number is close, say +/- 10000, to one of the breaks, it may belong on the other side. You can get a copy of Flayderman's Guide to Antique American Firearms to determine which model you have. This book will also give you an idea of how to grade the condition of the gun and determine the value.
Your Winchester model 1873,was made by Winchester in 1891.
Do a google search for "Winchester Model 12".
The Winchester model 20 was a youth .410 single shot gun manufactured between 1918-1924. Only about 24,000 were built. Your serial number is not from a Winchester model 20.
100-400 USD
$600
400.00 us dollars
100-300 USD or so
50-180
A Winchester model 20 shotgun in good condition(80%) original condition is valued at between 350-450 dollars.
That would be an amazing find. The 32-20 (32 WCF) cartridge was not introduced until 1882 for the 1873 Winchester rifle.
Lever action rifle
750 to 1200