An alpha character added to a model number usually indicates a minor variation (different sights, finish, or quality of wood, etc.). If it is a bolt action rifle, I would guess it is so similar to the Model 670 that there is no difference in the basic description or value so the guide books do not give it a separate entry.
If you list your rifles serial number in this question,we can determine the correct year of production for your Winchester model 1892 rifle.
IIRC, the BATFE Curio and Relic list might have the answer.
See the link below for a list of people who make a living repairing older air guns. Currently Winchester air rifles are marketed through Daisy airgun company. Winchester air rifles have been made in Spain, Germany and Turkey under the Winchester brand name.
Standard model list price: $99.95
You need to list the model number too.
http://armscollectors.com/sn/windates.htm
http://armscollectors.com/sn/winlookup.php?file=win1906.dat
The sn you list does not have enough digits.
The answer appears to be 1916. Try this website for a list of Winchester Model 12 Serial numbers http://www.mchenry-sc.org/Engineering/Model_12.htm
I can't find a serialization list for this model, but it's not old since the model was introduced in 1978 and is still in production.
I checked the entire list of Winchester model numbers going back to 1866, and there is no mention of a Winchester Model 39. The only two Winchester models with two-digit numbers beginning with "3" were both shotguns, not .22 caliber.
Not all books will list all models.