Nimbus Singlebolt – Slim Silver Clamp – 28.6mm – Silver
It is an aluminum single bolt seat post clamp from Nimbus.
A 22.2mm seat post uses a 25.4mm clamp.
A 25.4mm seat post uses a 28.6mm clamp.
A 27.2mm seat post uses a 31.8mm clamp.
Some Eclipse and Equinox frames need a 34.9mm clamp
These are a rule of thumb. Yet, variety exists.