Letters of JRRT Question

I don't have my copy of Letters where I can find it (blast) - does anyone happen to know the reference in Letters of JRRT indicating that Tolkien drew the relationship between Frodo and Sam from the WWI British officer and his batman? Wikipedia cites Letters as the resource confirming this, but it doesn't give specifics. I've got a standalone short fic I've just finished and am writing my notes for, and I'd like to provide specific details on this if at all possible.

Thanks for any help you can provide!

Thanks,
Febobe :)

A posting of interest... (invalid-cookery article!)

FrodoHealers and readers who recall my penchant for invalid-cookery might enjoy this article, as did I - especially as someone who, like its author, lives with autoimmune disease. :)

http://www.saveur.com/article/Kitchen/Victorian-Invalid-Cookery