Given the agent suffix -ter- (which exhibited e-grade when meaning a profession or purpose and o-grade when meaning the recent perpetrator), can -trom suffix also be a zero grade from this one plus inanimate nomenative ending -om? Especially taking into account internal derivation. Just some examples.
u̯es-tēr (tailor) -> u̯es-tr-om (wear)
a̯ero̯-tēr (plowman) -> a̯ero̯-tr-om (plow)
On the other hand, Fortson lists -tro- suffix along with -tlo- suffix as separate instrumental suffixes. I wonder whether then was a similar agent suffix -tel- (cf Russian prii̯a-tel "friend", Greek aristo-tel)?