What size of monocoque boom tube diameter does this Lockjaw boom head fit onto? I am thinking of retrofitting a HPL 160 carbon boom.

I have done these retrofits to HPL booms before, installing Maui Sails SDM boom heads. They work really well, but they don't make a RDM version, and I don't like to keep losing shims.

More on this.

Maui Sails retrofit boom head comes with shims to fit 36 mm and 40 mm tubes.

Chinook retrofit boom head fits on 33.5 mm tubes.

What tube diameter does the Severne Lockjaw fit on?

I tried extensive research for a retrofit ... I wouldn't waste your time.

Don't forget to check:
1. slight curves on tubes;
2. angles at each end of boom head;
3. changes in diameter across tubes
4. diameter as you said

I checked everything, bought a new head, spent ages with minor sanding to get a perfect fit .. never really worked.

Tolerances are tighter than you expect.

Wave 32mm diameter, 88mm width.