Having owned a few italian vehicles, including Moto Guzzi's where it seemd to vary from week to week whether the brake calipers were in front of or behind the forks, and bikes built on a tuesday had a the filler cap the other way around, I can well believe there is scope for variation. We are dealing with a minority RHD variant after all.
But my wife's first GTV, the T-prefix one definitely had HB3 main beam, HB4 dip.
Likewise the current GTV, a 52 plate, has all four HB3.
I know this because I seem to have changed quite a lot of bulbs in 100,000 miles.
I don't tend to trust the manuals where they differ with the hardware. I have an Italian bike where the English language section of the workshop manual has been americanised, including translating all the fluid capacities into cubic inches, usually with the decimal point in the wrong place!
In engineering, it is also not uncommon to use two suppliers, to avoid issues with availability, so i would not be surprised to see two different brands of headlight in the same year.
By the way, it's worth buying in a couple of spare bulbs from eBay for £2 a go, because they are £24 in Halfords!
I was going to ask what the brake and tail bulbs are, but maybe I'll just buy a selection, they'll come in handy eventually!