Being new to the process of manufacturing something (anything) is no excuse for shipping poor quality. It's a valid excuse for a slow ramp up, for high scrap rates, etc. But NOT for shipping poor quality to paying customers.
You are definitely not nit picking. On looks alone these should be returned, but I would also be concerned that welds that look so poor, may also be structurally poor, i.e. poor penetration etc because it is fair to assume that these were not welded by a competent / confident welder.
My hypothesis is that Norton deemed these unfit for fitment to a new production machine... so you got them instead! I realise that sounds bad, but not as bad as the thought that these ARE acceptable to production !
Whatever the reasons, you are totally right to 'reject' them, and in doing so, send a clear message to Norton that they have fallen short of the mark.
However, before you post them back why not communicate via phone and email, send these pictures and state your case. They might say "sorry, keep those for now and we'll send 'proper ones' later".