116v

[Anton]

We have something that looks like a grammatical ending here, so we should use it to our advantage in weighing the options.

In fact we don't. We have the five-letter sequence "pbren" (if we admit that the first one is "p", and the last one is "n") which does not make sense literally. Hence one seeks to expand it as an abbreviation. Once expanded, the abbreviation needs to be "grammatical", but an abbreviation as such is not grammatical. "USA", "UN" or "RUS" exhibit no grammar in them.

The leading "p" would be common for "pro-", "per-", the trailing "n" with a tail I don't know for what, unless it's rather "r" with a tail in which case it might be "-rum".