Although it is a long time since I rebuilt my Duo art Grand my
understanding is that the springs are of the same strength and that
it is the settings which vary slightly.

Making new springs is a doddle.  Just wind them out of steel piano
wire, of the correct gauge, on a lathe over a length of round steel
or brass rod of a diameter which results in the right spring diameter
when the tension is relaxed.

Their extension characteristics can be checked by hanging a suitable
weight from them and measuring their normal/extended lengths.  To check
whether springs are of the same strength by inspection just join them
in series and hang a weight from  them.

David Snelling - Isle of Man.