Ahh - Betas and bonnets! I think on that one there can be (and often is) a difference between theory and practice.
So here goes with the theory:
All S1 Coupes, Spyders & HPEs be they 1600 or 1800 had the flat bonnet with the narrow central ridge.
On S2 cars, the 1600 kept with the flat bonnet and the 2000 had the bonnet with the raised central section.
1300 Coupes had the flat bonnet .
S2fl1 and fl2 cars all had the bonnets of the 2000 (that is the one with the raised central section) with the exception of VX vehicles which had an additional little hump.
However..... in practice (and there is probably a factory chassis number change over reference somewhere) the later S2 pre facelift cars all moved over to the raised centre section bonnet before the car itself changed to S2 fl1.
Clear as mud ehh?
Is it at this point I mention that many years back, when Lancar cleared out all their old slow moving stock and gave it to the Club, we discovered two Beta Coupe / HPE / Spyder bonnets which were different again! They appeared to be the S2 fl1 type, but had a factory created 5 inch square hole in them, not quite where the supercharger bulge is on the VX cars. I don't think we ever did trace what they were for .... and I have no idea what happened to them!
Andy; I know when you had a look at my 1978 1300 you felt that it should have had the flat, rather than raised bonnet. Having checked the non re-sprayed underside of the bonnet with the un-resprayed area of the A post, which is only visible with the doors fully open, they are the same. Additionally, the bonnet isn't in particularly good nick, matched only in fact with the now replaced offside sill. So we have the scenario where the flat bonnet, if the car had one, must have decayed at a disproportionate rate on a car laid up indoors for nearly 20 (documented) years. The post above confirms that Frank broke a 78 1300 that also had a raised bonnet.
My theory, for what it's worth, is that at some point Lancia ran out of flat bonnets (in the knowledge that they were to be discontinued) and began fitting the raised version to all cars:- As you say, clear as mud!
Neil