Possible daft question but I've noticed on a few photo threads a number of On-Ones with "Inbred" on the top tube, but declared as 456's – were some 456's badged as Inbred for a reason or am I missing something? Put me out of my misery

456 is an Inbred but has a bigger diameter top tube so is a tad heavier and tougher. Angles are essentially the same but based on a longer fork for the 456, as far as I recall.

so, how do you tell which is which? As far as I can see recent 456s have that on the top tube but older ones said "inbred" on both models.

Don't know off hand but the subject has been repeated several times on here. Brant has listed the differences on a lot of occassions. Try searching using the advanced search.

I think it is a bit stupid as unless you know what you're looking at the two different bikes can be labeled the same. The TT on a 456 is beefier and, noticeably, so is the bit the rear seat-stays connect to – though for it to be really obvious you need the two together though. Or at least a picture of the other type to compare. Don't know if that helps, but they are quite different frames (different fork allowances, TT length and slightly different angles) but with confusingly the same name printed on them! (I'm glad the new ones have 456 printed on them!)

Easy really, all the 'proper inbreds' where white, the 456's weren't.

If it's made of 853, it's not a 456, if it's got singlespeed dropouts, it's not a 456.

I think

if the top tube is the same diameter as the downtube, it's a 456

The development of the 456 from the standard Inbred is explained in a fair bit of detail here:
http://www.on-one-shop.co.uk/?page_id=590