? There's the bracket for a kickstand and what appears to be a hole, but more likely a rust patch, pretty much I between probably where an inner chain wheel has rubbed.
Right, the "hole" here's a closer photo of it, kickstand bracket to right. It looks like a dedicated hole, for what I don't know. maybe something to do with a kickstand? If so, won't be using that then.
Got out the trusty calipers and the seat post is 25.4mm (admittedly the paint seems thick). Quite conclusively Gas pipe, as it's affectionately called here! Just the ticket for a fixie commuter .