The A-12 has a bore of 73mm, the A-13 has the same bore but additional deck height of 15mm. the A-14 ande A-15 have a bore of 76mm and the tall deck height. To make matters worse, the A-12A has the same short deck as the A-12 but the bore is increased to 75mm. All of the engines, other than the A-12, have the distributor near the middle of the camshaft. The A-12 has it up front by the oil pump. Hope this helps.
By the way, I'm not just a know it all, my new Nissan Motorsports catalog just arrived and I'm quoting it.
Also, I'd like to throw a big "Hello" out to my grumpy old buddy George Ward. Thanks for lurking on the site. Why not join in on the fun? It's free!
I can't spell sometimes, sorry about that George.