What I saw on my primary

I bought a 13.1-inch Coulter a few months back. I've heard Coulter made some good mirrors and not so good mirrors. I worked up the courage to take the base of the scope off - the mirror cell with the mirror. The cell was fairly new according to the previous owner. The mirror was secured with silicon or similar product. The mirror measured 5/8 inch thick and the number '#328' was written on the side with a felt marker. A minus 2, as in '-2', enclosed in a circle was also written twice with a felt marker. Now the question is, was this one of the better or not so good mirrors? What does the -2 stand for? Is it possible to date the mirror based on its number? I haven't tried high magnification, such as 300 or above. I was just wondering how this mirror would rate in the universe of Coulter mirrors. Does anybody know???

Thanks

David Roberts

The 328 is probably the serial number. There might be folks who can relate that to a vintage year. The -2 may mean the focal length was 2" shorter than nominal, so adjustments in the spacing of the secondary and primary had to be made accordingly. Mirror makers do not worry much about exactly where the focal length wanders, and mirrors often turn out a bit shorter or a bit longer than intended.