The originals are D1450: https://github.com/Ultimaker/UltimakerOriginal/tree/master/1127_Table_Spring_D1450
But I recommend stiffer springs then this. I've got 1 printer with the springs from the feeder, but those are too stiff. D2150 springs are the ones used in the UM2, which do their job quite well.
I'm not sure what specs the original springs are exactly, but here are the ones I used in my heated bed:
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