I see the question has already been answered - I was on my way out the door to work and didn't have a chance to answer. However, I grabbed my copy of a book I'd like to recommend to anyone who is interested in having a ready reference to such exotica:
"Photographic Facts and Figures," by E.J. Wall and F.I. Jordan, reviewed and enlarged by John S. Carroll. The printing date on my copy is 1976, but it had been printed and kept updated since about 1924.
I collect old books on photography and find them in general to be treasure troves of 'lost' and 'arcane' information that may become useful once more due to the zeitgeist. This book is a wonderful compendium of all sorts of information in one place - a great ready reference.
From page 74:
#49 Selective - Stronger than #47 (standard tri-color blue). Used for color separation from transparencies.
There are five blue filters listed with Kodak Wrattan numbers:
#47 - Standard tri-color blue
#49 - See above.
#50 - Selective - Used for black and white separation of weak colors.
#43 - Minus Red 2. Used to pick up red as black, while other colors come out light.
#44 - Minus Red 4. Stronger than #43.
The book further states that Wrattan filters were designed and designated as either "selective" or "corrective". Selective filters had sharp cut-offs in the range indicated - they do not pass very much of the other colors. Corrective filters pass more of the other colors than selective ones do.
I bought my copy of this book for $5 or so on eBay - the come up all the time. I'd recommend it to anyone who does not already have a bookshelf full of ye olde Ancient Wisdom. If you only buy one such book, this would be a good one.
Hope this helps, I realize it is more than you asked for.
