This special Spacer measures 12mm (id) x 16mm (od) x 16.5mm (long) & is known as a Bearing Support Spacer. This Bearing Support Spacer fits between the Bearings, along side the 10mm O-Ring when fitting Bearings into a TRAMPA hub when mounting the wheel onto 12mm Axles.
Over time, during the usage of your board, the bearings & spacers are put under a lot of force. Ultimately this force has to go somewhere... The Bearings use a Steel ring in their assembly where as the Bearing Support Spacers are made from Aluminum. Steel is much stronger than Aluminum & when forced against each the aluminum will loose out every time! This is what happens to your spacers between the bearings, the Bearings put force onto your spacers as a consequence over time the spacers shrink... With enough time its possible for an annoying rattle to start.... That's when you fit these 16.5mm spacers with the little 10mm O-Rings to prevent the rattles!! :-)