Hi Mark,
Let me say first off that I do not have a rim for you. But because I have a 58 Lenton GP, I wanted to reply. I love mine - what a great bike! Very versatile.
Have you seen this excellent article that list member Peter Kohler wrote on the Raleigh Lentons?
http://www.sheldonbrown.com/
Since you mention a left-hand shift lever, I'm wondering if possibly your Lenton is later than '58? Mine was built in '57, then assembled and sold in '58 as a '58 model. It still has the rod operated front derailleur - you can see pics of my bike here:
http://pg.photos.yahoo.com/
In Peter's article, he says, "The 1961 models had a cable operated front derailleur instead of the rod-control."
I suppose it's possible someone retrofitted yours with a left hand lever? is there a cyclo boss on the left side of the downtube?
Either way, I'm sure it's a lovely bike - and I would love to see photos, if you could post a link.
I know what you mean about the size issue, too. Since Raleigh only made these bikes in 21 and 23 inch, I ride a 23 inch frame even though it's too small for me (6'2"). I just show a lot of seat pin!
Best Regards,
Duncan Granger
Mountville, PA