Matt,
That is interesting.
I've not come across the 145x59 section before.
The other one is the standard section.
My understanding is the same as Wouter's in as much as there were some cat rigged sections but likewise I was of the understanding the only difference was wall thickness.
Unfortunately I have never actually measured one to confirm.

Regards,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!