I'm guessing no one. I hadn't seen it before this thread either, just suspected it was an older convention (and so Googled) because it reminded me of a formal way of signing off I'd once seen in a German letter.
I think he's actually used what used to be an acceptable construction in formal letter-writing, though it seems to be less common these days: 'with such and such/in such and such/thanking you, I am sincerely yours'
e.g.
+ other examples on Google: https://www.google.com.au/search?q=%22i+am+sincerely+yours%22+letter