classic motorcycle forum

Correction first: Above I said that KZ was the prefix for the 196cc Brooklands engine; I meant Super Sport. (I don't think there was a 196cc Brooklands?)

Anyway, thanks for the photo of the engine. Two thinks are very clear, both of which you mention in your first post: the boss on the head is not machined for the decompressor (in fact, the head looks a bit like a recent casting?), and the boss between the exhaust ports is not machined for the oil pipe that comes with the Villiers automatic oiling system. Reading Browning, it seems that this connection, when present, is for the oil to go in via internal drillings; the pressure is provided by the other connection lower down on the crankcase.

Browning lists the 196 Super Sport prefixes as KZ and KZS, without saying what the different prefixes refer to. The VMCC machine register lists 3 KZ prefixes, 7 KZS and one lonely KZB (KZB 398) all fitted to FBs 1929-1930-ish. I guess the prefixes refer to combos of oiling system and lighting coils, with KZB being petroil and no lighting? Let us know when you figure it out!

I assume most/all KZ engines had the decompressor originally (see photo), but I doubt it's really necessary. Most useful for stopping the engine?

Cheers

Leon

Some clarity needed here
He is talking heads and you are talking cylinders 

Might be a slip of the pen.
But that photo would help ...

That's interesting. I think we might need a photo.

Most Francis Barnetts used the 172cc Super Sport engine, which has prefix BZ. The hot Brooklands version has prefix Y.

KZ is the prefix for for a 196cc Super Sport [edit: not Brooklands as I first said] engine, but I'm not familiar with KZB... Maybe it's some variant on the Brooklands/Super Sports.

The drilling in the cylinder between the exhaust ports is for an oil feed. The Brooklands head has a boss for a decompressor, but it is usually not fitted. Perhaps super-sporty riders/racers made do without a decompressor. If it's a huge problem, maybe you could use a thin aluminium gasket under the cylinder (not too thick or it will change the port timing) or a thicker head gasket to lower the compression a little.

Leon

I'm set !
A Bantam ? tin bracket and later (?) Lucas lamp seem made for the job.
And that aforementioned small 6v battery.

Have a few bicycle ones. Decided there was no mileage in them = dry cell batteries again.
A better quality of rechargeables might work, but have proved elusive.
And look notably modern.

I'd still be curious what the model of this light switch is ??

they originally gave but wangling a replacement in and out of that headlight clip every month or so would drive me nuts.

You and everyone else, so after the one BSA supplied (if they ever did) went up the swanee doubtless they were never replaced. Just park off-road or where a copper isn't going to visit.
If someone was poor enough to be riding a little two-stroke in the 1950s, wasting money on dry-cells would have been well down the list of necessary expenditures.