Chuck
I may have confused things by use of terminology so I'll summarise the arm rest situation for 6 cylinder cars. There are basically 3 types (excludes of course the BJ8). Most people will assume there are only 2.
1. In 1956 when the 100-six was introduced it had an arm rest similar to the 100's. That is the 'thinner' type as you call it, folded over the tunnel and secured with four tenex press studs. These original saddle type armrests were quite deep and covered the tunnel down to the floor with an arch piece cut out to fit round the handbrake. This type is clearly visible in early photos of cars being road tested etc. The artists impression in the old brochures also shows this to. This type is sold by most suppliers.
2. At some point, probably in late 1957, the arm rest changed to the type shown on my site under Longbridge interior. This is still a saddle type armrest, but is not as deep and does not have, or need, a cut out arch to fit round the handbrake. As you also noticed it fits further back than the first type as it butts up against the rear seats. The first type didn't. Few people were aware of this second type and thats why it doesn't get manaufactured by anybody.
3. The third type is the 'pad' type. This is the 'fatter' type you mention which is I believe sewn onto the carpet on top of the tunnel. It is smaller in area and does not fold over the sides, it just sits on top. Not as nice IMHO. All 3000's have this type as do the 100-Sixs made after Sept 1958 that have the 3000 type interiors and detachable top (hood for us Brits).
Now the question. I used to think the pad type was introduced on the BN6 but now I dont think thats the case. The BN6 brochure shows a saddle type, but the problem there is that you can't trust the artist impression brochures, This one is just adapted from the BN4 brochure. But having studied closely some older road test pictures of the BN6 it does seem to have the saddle type armrest of the later type. (2 above)
I believe your car should have this later saddle type and not the fat pad type (sorry if I mislead you earlier but I've learnt something new which is good. Thats the good news , The bad news is I dont think anyone supplies them so you will have to have one made or go with the earlier saddle type which is readily available and I think, looks and fits better than the later saddle type anyway.
hope this resolves the issue. Im glad you brought it up
cheers
Derek
www.healeysix.net