Howdy
That one looks to be in very nice condition. That style of service grips was generally used from about 1920 to about 1929. The one I can see looks to be in beautiful condition.
I have two of them, both of which shipped around 1917. Notice the different style grips, this style with recessed 'gold' medallions was used from about 1910 to about 1920. I can't quite tell from your photo, does yours have the large 'mushroom shaped' cap on the ejector rod like mine? If you take off the grips carefully, you should find the serial number penciled onto the underside of one of them. I always forget which grip, I think it is the right grip.
This is a comparison photo of a K-38 from 1950 and one of my 38 M&P Targets. Notice the tiny little adjustment screws for windage on the rear sight. You need a tiny screwdriver. You back one screw out, then use the other screw to shove the blade the direction you want it to go. Then you snug up the first screw. They are delicate, don't over tighten. The modern 'micro-click' rear sight first appeared in 1940 on the rare, pre-war K-22 Masterpiece. First used on the K-38 in 1946.
$100 for a factory letter?
Ouch. Don't hate me but I remember when they were $30.
Frankly, I don't bother to letter a S&W, particularly at those prices, unless it is something unusual.