Amongst the spares that came with my CT3 was a pair of original rear shocks, lucky me.
The shocks are damping fine and have minimal surface rust, which is currently melting away in some Evaporust.
The chrome on the springs is terrible and re-chroming is not an option because of the cost and the risk of hydrogen embrittlement, so I've been looking into alternatives.
I've checked the spring rate and it seems to be 12.5N/mm.
These YSS progressive springs are dimensionally very close and I can select the 13-18 N/mm spring rate, just wondered if anyone had tried them or other springs.
www.suspension-store.com/46-13-18-200-(02)-Chrome

YSS are making quite a name for themselves. No one heard if them a few years ago, but they have made themselves quite a reputation for good quality, affordable kit.

Cannot help you with your question.

However I can tell you that Brook Suspension in Bradford are YSS dealers. Maybe a phone call to them may get you the info you need.

www.brooksuspension.co.uk