Maybe extra finish was a way of making your stocks more impervious to the elements more so than an item of luxury. Kind of like using a synthetic stock today as opposed to wood. More a thing of utility in a day when stocks were restricted to wood. Seems to me if you were going to push a high grade stock finish to adorn a high end 99 you would use a term more exotic term "extra." Something more like London or French.
You may be on to something. Although before redoing some stocks in varnish I didn't know how thick to apply them, so experimented on some test boards. The more you put on, the easier it is to harm/break the finish. Maybe the normal varnish finish was just enough to fill and coat the stock, and extra would give the deep glossy finish.
