This is Louet's Standard Bobbin Lead (Irish Tension) S10 Bobbin. It fits all older model Bobbin Lead wheels.

Capacity 6-9 ounces
Ratios are: 1:5.5 (slow spinning/heavy yarns), 1:7.5 (medium yarns), 1:10.5(fast spinning/fine yarns)

Fits wheel models S10, S10DT, S51, S51DT, S15, S17 and S75DT.