when you say "75D", you are using "D" to stand for durometer, not the hardness scale.
Elastomer hardness is typically measured on either a "Shore A" or a "Shore D" scale. The "A" scale is 'softer' than the "D" scale. The two scales overlap.
A comparison of these scales can be found at:
Don't mean to nit-pick your post, but didn't want someone who wanted a 'soft' o-ring to order a "40D" hardness o-ring when they were really after a "40A" and end up a hard one similar to a "90A".
Other examples of hardness of different elastomers can be found at: