Spigotted barrels?

General Engineering) a short cylindrical projection on one component designed to fit into a hole on another, esp the male part of a joint (spigot and socket joint) between two pipes.
in this case the spigoted barrel fits inside a cylinder head recess .  Instead of the cylinder head sitting on the top of the barrel,  surface to surface they engage inside each other giving a more exact location.  In single cylinder machines the spigot is ground slightly higher rpthan the depth of the socket so that when it is pulled down by the bolts the seal is made between top of spigot and inside of cylinder head. 
I have no knowledge of the lightweight twins….
Jon

See attachment for comparison of spigotted and non-spigotted Dominator twin barrels.  The two 99 barrels both have top spigots which sit inside respective recesses in the cylinder head.

The spigots act as a barrier against combustion gasses and help to prevent (supposedly) the head gaskets from leaking oil. On non-spigotted cylinder heads the gaskets must be all-metal or have quality flame rings to do a similar job.

These are different part numbers because the Electra lower cylinder spigot is slightly larger diameter than the Navigator lower cylinder spigot.

Plenty of Navigator lower cylinder gaskets around.

If you can't find an Electra lower cylinder gasket, take a new Navigator gasket and fit it over the 12 cylinder studs of the assembled crankcase. Craft knife or scalpel blade will shave the gasket until none overhangs the crankcase bores. Try not to shave the aluminium. And somehow make sure the gasket shavings don't get into the engine and block oilways.

Peter