I have seen a couple of these. One sold on auction recently for $575, and another is listed on Gunbroker.com for an opening bid of $995, but it has not yet had a bid.
Receiver
On the receiver
It means that it is a Model 1890 (90) receiver. When Winchester introduced the Model 1906, they used the exact same receiver and action as the Model 1890, with exeption of the cartridge carrier. To readily differentiate between the two, Winchester stamped an "A" on the Model 1890 receiver and a "B" on the Model 1906 receiver. Bert H.
Check the receiver.
Look at the receiver
Receiver/barrel
Check the receiver and barrel
Receiver.
Receiver
Receiver.
Bottom of the receiver
It will be stamped into the side of the receiver.