Question about staybolt maintenance on the N&W

[nw-mailing-list at nwhs.org]

I'm hoping someone on the list can confirm my memory on this one:  I'm 
certain that I've read the following in at least one article on N&W steam: 
the N&W averaged only 3 broken staybolts per locomotive per year during the 
heyday of its steam operations (1940's/1950's).

I have an aquaintance in Australia who is doing research for a book on steam 
locomotive boilers and he was very interested in this statistic when I first 
mentioned it, but unfortuantely I've been unable to back it up.

I made a concerted effort some time back to find the source of this quote 
("Trains" articles, "Railfan" articles, "Giant of Steam", other N&W books) 
to no avail.

If anyone can confirm this statement and the source, I'd really appreciate 
it.