Herman MillerClassic Aeron Chair Lumbar Pad - Graphite - Size B

How do I determine whether my Aeron chair needs a size A, B, or C lumbar support pad? My chair's lumbar pad has 17 grooves and is about 11-1/4" between attachment points on the rear of the chair.

Is there a way to check the sizes of the A/B/C/D . Unfortunately I can't find where its indicated in my broken one to replace it?

Does anyone know what size (i.e. a, b, or c) a 12.5" lumbar is? it fits a b chair but i have to bend it and force it into place. that doesn't feel right.

Is this herman miller genuine part #133760 for aeron classic type b?