Does anybody know of any such thing??

None???
Not even in regular blued steel either??

The closest thing you might find is that Brownells offers a Sunny Hill brand Drop magazine box/trigger guard assembly for the pre-64 in >375 length. I bought one several years ago (when they were much, much cheaper) to put on a >375 pre-64 I have and it is really nice. Anyway the going retail price of it now, according to Brownells #55 catalog, is around $461.00. You might be able to get it cheaper if you know a dealer who orders from them. I looked for a long time and short of having one made by a machinest, this was it. Now, you might try a having a machinest do one for you. Not necessarily a gunsmith machinest, but any machine shop can make whatever you want from a drawing. Hope this helps...