The letter has no real meaning. When 73’s came out they had just a plain serial numbers then in 1882 around serial 89,000 they started adding the A  and then it changed to a B. The B started in about 1884 around serial 171,000 and intermixed with the A’s into 1885 around serial 195,000  and continued until to the end of production with the B.

Winchester did not mark the caliber on the 73 since at first there was no other caliber so they didn’t need to mark the guns. Then after they started to introduce other calibers they marked the new calibers but the 44’s continued to be unmarked for a while.

Bob

To add to what Bob stated. Your 73 having been made well after 1879 when the 38-40 was introduced and also most likely being a 3rd model would have been and should be marked for its caliber on the elevator and barrel. Granted if the rifle has much use then the markings very well could have simply be worn off. Even if the caliber marking is worn off on the top of the barrel and elevator, it would most likely still be visible on the underside of the barrel underneath the forearm. If it is not, then the barrel is likely a replacement. 

Sincerely, 

Maverick

Your gun was made before they were proofed marked so if you have a proof marked barrel and receiver, it went back to the factory. Originally it may of not had caliber markings on it. It wasn’t until 150,000 that they consistently marked 44’s

Bob

I’m going with that you markings have worn off. Especially as how hard it is to read the serial number. Looks like those adjacent hash marks on the elevator maybe part of the “44 wcf” marking remaining.