Jus a warning Martin - that horn looks like a more modern type.  It could be a 12V.  Have a look at a bulb in headlamp or tail light to check.  Within a few years of your Model Norton did convert to 12 V so the parts are compatible. Best to check.   Good luck, Howard 

This machine looks original with the 18D2 and would have been 6V with the PRS8. These machines were never 12V from the factory. But they will readily convert. Do think about the conversion before buying a 6V battery. Don't worry too much about the numbers the battery box will take quite a large battery, just get in the largest you can. Do NOT use Alarm or cyclons while running on 6V alternator switched (PRS8), for final road use BUT you could use anything to get it going, perhaps even a run round the block.

A note on buying motor cycle batteries - when buying mail order you may find they come dry without acid and you end up paying a garage £10 to fill it so negating the saving you thought you made buying online.  The bike looks very smart BTW.

Thats a bit of a quandry , you want to try it out but dont want to waste your cash on a 6 v  battery .  But the bike will not run without it. I was at the Council tip dumping some batteries in the recycle box. There were every type of battery in there ,Some obviously new ! . I rescued a new 18 v Makita one. Now i have 2 good ones !.Take a £5 for a tip. And a volt tester?. If you switch bike to 12v you can then improve ignition with addition of expensive or cheap electronic ignition . led bulbs ,cheap rectifier ,.2MC cap etc. 

Yes the Recycle Tip has endless items the average person throws away but is actually in good order. But you have got to do a bit of bartering! In this case just about any 6V battery, even an ex VW battery on the floor could get the bike running.
Buying a battery from the Internet?? If it goes wrong then returning it will be very expensive, cheaper to go with a local supplier for a decent battery, but a cheap alarm battery will get it going but any distance will kill the alarm battery, 
But OHH! those handle bars! If you are over 60yrs old and live in a town-they will kill your back!!!
A pair of straight bars before you go any distance. This is NOT a cafe racer!

I find it interesting that the 6V Lucas batteries for many old bikes are often quoted as the Model Number above, whereas the correct number for the battery should be "PUW7E" - the  "Z" indicating dry-charged for export.  Possibly the "W" meant "Wet"? What I also find even more interesting is that the old Lucas battery was rated as 10Ah at the 10 hour rate, and 14Ah at the 20 hour rate. Quite a decent capacity when you consider that many people seem to think that 7Ah is the modern norm. So my advice, if sticking to 6 Volts and want a battery with some guts, is to go for a Lucas L6N11A3A1. They are 11Ah at the 20 hour rate.  Currently on eBay for £25.99 inc delivery. If you want a Gel battery there's the Atom 6N11A-1B for about £27.  As to whether acid is included, most of my experience is buying car batteries by post.  They are always complete with acid.  I did buy a 6V one once for my Navigator and the acid was in a separate bottle.  The main difference would be down to size - and weight!  The biggun's are much easier to keep upright!

Mister Pedantic has now left the building! :-) 

This battery is fine for keeping the bike 6V and not going to Electronic ignition.
Obviously later changing to 12V this battery is wasted.

You've already bought one, so my reply is a bit late. But just to add my two pennies worth to this topic for future reference:

I've kept my bike 6V and it had a Cyclon lead acid battery on it. One of those with 3 round cells with rolled-up lead. They seem very popular in our bikes, although they are officially for emergency lighting applications.

The Cyclon was knackered due to it having been discharged for years. I bought another one and that worked just fine. But because I let it sit in the bike without charging during winter, it went too. I reckoned it was my foult and bought the third one. Did charge it every couple of weeks next winter and it ended up dead too. I now have two small lead gel batteries in parallel and that setup has been great for the last two years. They're very cheap too.

Cheers!

Tim

Tim, you haven't said what the bike/charging system is? If it is 6V alternator-PRS8 switched then the cyclons will fail-over charging. IF they were charged with a dynamo with a poorly set up original control box you could have had the same result, ie over voltage/over charging as cyclons are fussy. If you have a decent voltage control then they might survive.
One other point here is it is always a good idea to have as many AHR in the battery so if you can get 11AHr in the Dommie BBox then do so.

You can no longer get a "dry" battery and a bottle of acid to fill it yourself. After a spate of attacks with "corrosive substances" the law was changed to regulate the issue. Can't give you chapter and verse, or what constitutes an "authorised person".
So AFAIK, even if you obtain a dry battery, as mentioned above you'll have to pay for it to be filled commercially. 

The Cylons failed with a dynamo and control box. I can't remember the values, but I did measure the output. It wasn't that, but I don't know what the problem was. Maybe bacause I had the Cylon on its side. I know many are perfectly happy with them, so it could be my bad luck or ignorance.

And you're right about the AHR Alan. The more you have, the better.

Still, I'm quite happy with my two coupled 4Ah gel batteries. It's not much, but it's enough.