There's a good little book called "Colt New Service Revolvers", by Bob Murphy, that is about the best collection of NS info I've come across. The books by Wilson, Servin, Haven & Belden, and a couple of the newer books contain varying degrees of information on the New Service, but Murphy goes into much more detail. I bought my copy shortly after it was printed in 1985, and have only seen a few copies since. You might try a search on bookfinder.com
Murphy's book is still currently available, "The Gun Report" magazine sells them for $7.50 plus postage. The Sutherland & Wilson book, "The Book of Colt Firearms" has about 6 great pages of info and pictures. The book is expensive but if you know someone who has it get photocopies of the 6 pages on New Services and Shooting Masters. These two sources are the best,and I have all the others mentioned. Good luck...
Can anyone tell me what the letters after the serial number on the New Service mean. example 345**D and some have only the serial number 878**. Is it where they are manufactured others have L H B.