273 Engine with Frankenheads Setup

I have an engine that is supposed to be a 66 273, that has the early (65) casting number heads but the heads have the larger bolts and late bolt angle and no sign of modification. Only thing I can figure is that they machined some of the early heads with the late bolt angle at the beginning of the 66 production run. Yours with mis-matched heads sounds like it has probably been worked on.