Does anyone know why Leyland marked up some of the components with one
digit higher than the official part number? Was it to confuse the Japanese?

Examples:
From the main site, Mark has got hold of this excellent blueprint for the
12G295 small bore head
http://mk1-performance-conversions.co.uk/12G295_drawing.pdf
The drawing quotes it as 12G294...

The C-AEG648 camshaft is marked up as "649"...

There is a passing reference to this practice in the Vizard book, but I have
never seen an official explanation. I suppose that in pre-computer days,
it would not really matter as any documentation would be paper based and
it would be easy to find, even if "filed" one digit wrong. In this day and age,
you would probably just get "Computer says 'No'..." as a reply.

Ian

it was always done this way (on cast stuff)

the part number is the id of the current evolution of the part (or at least the 'finished job')

the cast number is the id of the casting

i.e a 12G294 is the bare 'dumb' casting

the 12G295 is the machined item.

they wouldn't renumber the bare casting everytime they changed something other wise they would end up with literally hundreds of casting part numbers for very similar items - just look at the factory blueprints for how many minute changes occur over the life of a casting

it was also probably done this way from a costing point of view - i.e a 12g294 cost '£10', where as a 12g295 cost '£100' as it took into account all the machining costs.