A great thank you, usagi! The search may be even more difficult than I thought.
I may have read the paper more than 20 yrs ago, and the article may have been decades earlier.
I think I read the paper in a journal like Transactions of the Philological Society or possibly Bulletin of the School of Oriental (and African) Studies. However, I used to keep lists of titles of potentially interesting papers in every journal issue I consulted, but no relevant paper by Burrow (or Bailey) seems to be on my lists for these reviews.
The problem is about an apparent 2nd person bhūḥ 'thou wast' (or bhūs, according to what conventions are followed) used as a 3rd person 'he was', at least according to Burrow. Thus it has nothing to do with Hittite h or the nominative bhūḥ 'earth' Substrate in Sanskrit.
Since this is a personal proposal of Burrow's that perhaps no-one has followed since, there is little hope of finding a reference to it in a reference grammar.
At least I found no reference to that 3 sg bhūḥ in the basic grammars of Vedic by Renou of even of Sanskrit by Burrow (admittedly, both grammars' first edition is perhaps older than the paper in question).
I had searched the web for hours before posting my query.
Perhaps the only solution will be for me to look into two or three dozens of years of Bibliographie Linguistique…
Best wished to you.