Con,

Here's a scan, in pdf format, of an article by Ken Waters about the .350 G&H Magnum. The original date of the article isn't stated.

http://www.bunduki.com/bob/articles/350GHMag.pdf

In the article, he states that it is simply the .375 H&H necked down. Unfortunately, there is not a cartridge drawing in the article. Anyway, I thought you might enjoy the article.

Also, here's a scan of a page from my reprint/copy of the Griffin & Howe 1930 catalog showing the .350 G&H Mag:

http://www.bunduki.com/bob/articles/350GHMag-GH1930catalog.jpg

Quality Cartridge here in the States makes brass and ammo for the .350 G&H Mag:

http://www.qual-cart.com/350_G-H_Mag.htm

Cheers!
-Bob F.