Yes, the ‘box I’m talking about has some of these features but not all. The case is a different design, but it’s basically a standard three speed box stood on it’s end with the mainshaft uppermost. It does have the double row bronze cage for the high gear but (I’m pretty sure) the layshaft is solid as you say and not built up.The mainshaft has a larger diameter at the cover supporting a larger bearing. I had to have the relief around the mainshaft bearing’s bore in a spare standard 3 speed cover welded and then I re-bored it to take the new one. They were excellent ratios once you got going and although I’d spent a lot of time to find that it just wasn’t going to work, it was an interesting exercise. I run vintage close ratios now and though they still give you a high first when you have to gear up for a long straight, they are at least usable in all areas. Roger used to run the hybrid set on my bike.. I can’t remember the name (is it modern close?) or the mix, but the sprinters used it. First and second close together and a bigger gap to third. I couldn’t get on with it. To be honest one of the biggest performance mods I’ve done recently is to improve my clutch by fitting gauge plate plain plates that Roger has, each of which I had to dremel slightly to get to fit with between 0.005″ and 0.010″ clearance on the centre. Well worth an hour or two. With three gears on a circuit with a long straight it’s going to take a beating at the start and the more metal there is in contact and the less hammer there is between surfaces the better it is. My clutch is superb now, ultra light action and quiet. The great thing about using gauge plate is that you don’t have to wind your springs in just to flatten your plates to get drive as they are flat already. You only have enough spring tension to do the job they should be doing. I think I digressed…
