As Chewy says, most likely an Avon Sportboat. I've attached a photo of my S250 (3.2 metre length)
They come up regularly on Ebay as well as Boats and Outboard etc, so don't be too downhearted at missing this one. for example:
They generally last very well, but there are some dogs out there.
The maximum outboard varies between models, irrespective of length. For example the S100 and S250 are both 3.2 metres long, but one has a 15HP maximum and the other a 25HP maximum. For what will now be a pretty old boat, I would keep the engine size down.