We have a hydro Deluxe, with it's bum up in the air a little bit.
So thinking the spring has stretched a little bit.(But can work around that)

While looking at a spring, have noticed a little bit of orange plastic on it.

As the Cooper S has different hydro bags and the Cars sit 1 1/4 inches lower
than Deluxes.
Does this mean the Cooper S rear spring is 1 1/4 inch shorter than Deluxes ?

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I don't think so. Check the front knuckles haven't worn through the nylon cup

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The bitterness of poor quality (or Crappy Workmanship) remains LONG AFTER the Sweet Taste of Low Price is forgotten.

Cooper S front bags have about 10mm more thickness in the rubber cone above the bag.
Rear helper springs are the same as yours.
I have S front bags and stock rears in the wasaMatic and it sits level with 2 full tanks.
To level yours a bit the correct method is to fit a washer on the front knuckle joint pins. I think 4mm is recommended maximum.

Edit
All hydro cars were higher in the rear by design. But it gets exaggerated with age.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Check the hydro pressure as well. The bum will be very high if the hydro pressure is low. The front is the last to come up when pumping up a system.

It is a common problem to have the rear sitting too high on hydro cars and often, after a rebuild.
The ride height of the car will usually level out a bit once the car is moving.
Orange coloured springs were the final modification from mid 1966 for all hydro cars whether standard or S.

2 solutions I have tried are:
made up a pair of rear struts that were machined slightly shorter, when I had access to a lathe; and had a pair of adjustable rear struts made - similar to the adjustable front tie rod system.
Begin by using spacers on the front ball struts as recommended above. The factory suggests .150 thou or 4 mm as Drmini says.