Looks to me like a std retaining collar, similar to an axle bearing retainer. If no screws can be found on the lower pulley shaft, then Id say the pulley unit slips over the shaft, then held in place by an interference fit retainer.....But as grumpy says check with someone in the know. If it is the retainer, then it will have to be heated and/or chiselled off...But again, please get confirmation before trying to remove it, as we dont want damage to the shaft, or YOU:)
