Stem has definitely been used at some point. The witness mark on the quill and the colour fade are indicative of being mounted.There are enough chips in the paint and ingrained dirt to suggest so. Possibly off a Zolatone Eldridge Grade?. Centre to centre looks longer than 100mm, which would be unusually short for that era.
The 'Onza' is just a sticker, most likely added after a bar-ends purchase, nothing to do with the stem.