Dawson,
I may have already answered this on the other board but here goes...
Japanese barrrels will fit on recievers made for the same length chambers. The only issue with fit is caused by rubbing between the forend wood and the barrel because the Japanese barrels are a little beefier. This can be relieved with a litt sanding of the hidden part of the forend where it touches the barrel.
The reference about different barrel extensions is on VERY early guns. I've only seen one barrel with that issue and it was made in the first or second year of production.
Just for grins I shot a round of skeet with a Japanese barrel on my 1906 A5. It did not prove anything....I just found it amusing.
Jeff
