OWS (Outer Wheel slider) is an excellent upgrade from traditional sail slides to reduce friction when hoisting and dousing the main sail.
The depth and the shape of the slug is designed for earlier models of Seldén and Kemp sections (E-sections and D-sections used 1977-2002). The four wheels roll on the aft side of the mast section while the slug slides inside the luff groove. 

This car will also fit a great number of mast sections from John Mast, Soromap, Sparcraft and Z-Spars. 

For all technical information - https://support.seldenmast.com/en/products/sail_hardware/ows_sliders.html

To acheive maximum strength, the OWS slider should be attached to the main sail using a webbing strap. All sliders for webbing can be used for headboard attachments and as intermediate sliders.

H: 55 mm
D: 8 mm
E: 21 mm
SWL+Horizontal: 2.50 kN
RM: 75.00 kNm
FitsLuffGroove: 5.50 mm
Z: 5.80 mm
X: 11.20 mm
T: 4.40 mm
Y: 11.00 mm