Indeed the one in the first link is unsealed. It's only the single row angular contact bearings, unlike the double row, that are only available in unsealed varieties.

However, double row angular contact bearings are available in the sealed type (2RS).

These bearings perfectly as they take radial, axial and tilting loads very well - ideal for what the sprocket carrier bearing has to deal with:

https://www.bearingbasement.com/product/angular-contact-bearings-b-double-row/3205a-2rs1tn9-and-mt33-bearing-skf-double-row-angular-contact-sealed/

http://www.bearingsrus.co.uk/3205a-2rs1tn9-mt33-skf.html?gclid=CL2Xx-eA18QCFWbKtAodhQUAGQ

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