Latin -bs : just written or phonetic?

From: Joao S. Lopes
Message: 67921
Date: 2011-07-22

What can be said about Latin nouns with bs-endings, like urbs, orbs, caelebs? The Nominative suffix -bs was just a written form of an older *-ps, influenced by oblique cases, or represent a <bz> spelling? PIE have consonantal stems ending in labials?

JS Lopes