You are correct. There are no letter prefixes to the serial numbers in that period. The Serial Numbers that I referred to in my post above are not to be confused with the fitters numbers that you correctly identified as inconsequential to identifying the handgun.
To see the serial number on the back of the ejector star without removing the star, you have to shine a light through the cylinder chamber. That number is quite small.
To see the serial number on the back (the side facing the cylinder)
of the yoke, you have to remove the yoke and cylinder by removing the yoke retaining screw on the front of the trigger guard. If you do that, be very careful because there is a spring loaded detent in the yoke that assists in holding the yoke open. It is just one of the very fine fitting details that the S&W Factory included in guns of that era and often the detent pin or the detent spring is lost by people who don't know that it is there.