Imperial Threads on a Harrison 140 Lathe

I have a 1971 Harrison 140 with a metric Norton gearbox and thus metric leadscrew. The L5/L5A/140 manual suggests what set of drive gears to substitute to obtain 8 to 60 TPI, but gives no clue as to what the gearbox settings might be, other than giving the clue "… as for standard Norton Gearbox".

The complication is that the "standard" gearbox (Imperial?) had two thumb levers, rather than the three of the metric box, and might have used a 4TPI or a 6mm pitch leadscrew, i think.

I will measure the output with a dial gauge on the saddle, but before I try to buy the conversion gears has someone been here before and can they tell me what works?

Sorry that this is interminably boring for anyone without a Harrison!

Oh it was sad watching this poor posting slide down the 'latest posts' table with no one to answer or even commiserate Dear Moderators, is there a booby prize for anyone who receives no replies?

I should have guessed as even the great Tony @lathes said nobody knew; and people on the (slightly USA configured) Yahoo Harrison group site had raised the subject but not arrived at a full answer.

But I don't want someone else to search this site in years to come and think there is no known answer - I have now worked it all out. I fitted some gears, measured the travel with digital callipers, and found I could get 8-60 TPI from the Harrison Metric gearbox. If there is ever one other person in the world who wants to know then send me a message and I'll be happy to pass on the information.

If you put your answer in here then it's bound to come in handy for someone

Plus your particular solution may actually be relevant for people with other gearboxes

A lot of lathe doods really struggle with this stuff (hence the vast thread of replies)