I mistakenly thought all D14/4 had centrally located spark plugs, I am sure I saw one in the 70s but it could have been a D175, you can find better examples for originality that that on display for less money. Late bikes may have been a mishmash of parts, 1969 onwards, but not as much of a mishmash as the example shown!
Later D175, 1969, have a central axial spark plug location, it made timing easier, that head looks like a D7 or a D10 & possibly a D14/4;
http://bantam.150m.com/history/index.htmlThe Bantam was fun to ride a bit like a two-wheeled version of the Morris Minor, it handled well & the Bushman & Sports models had beefier front forks, worst problem was a tendency to four-stroke & the engine would occasionally turn over backwards giving you a involuntary reverse gear. Check ebay and the site I listed for information!
Grey porridge? Motorcycles? Put some sugar & honey on it!
Cheers
John
JBW