Russian Empire

I have always played around with collecting Russia 1845 - 1917. I just recently got a little more serious with my collection and have a few questions to throw out there.

1. I purchased a catalog by Zagorskiy And there appears to be separate numbers for watermark varieties that Scott does not list. It seems that certain sets were issued in different years with the only difference being a vertical or horizontal watermark ?

2. Does anyone know of a good dealer of these early stamps?

Thanks for the help.