Good morning. I was told that the measurements didn't come through clearly in the pictures, so I remeasured and this is a 56. It's squarely 56cm, measured center to center. When I posted yesterday I misremembered and unfortunately I can't change the subject line.
It is a 56cm. The seat tube measures 56cm CTC and the TT (CTC, but there's a 2* slope) is about 562. Riv's geo chart is here:
https://web.archive.org/web/20170602082329/https://www.rivbike.com/pages/geometryAnd here's the geo on BikeInsights (comparing the 56 and the 58):
https://bikeinsights.com/compare?geometries=5a1e69902a0c030014708caa,5a1e69902a0c030014708cab,Finally, here's the product page from that same archive/same era is here:
https://web.archive.org/web/20170704042854/https://www.rivbike.com/collections/framesets/products/a-homer-hilsenThe only picture with the measuring tape I can discern is the headtube (~246mm), which is not a helpful measurement, IMO, but I know a lot of people look at that like gospel.
Daniel in LA, CA, USA