I've never seen that on one of the older Ferrograph decks, but it's a common problem with their 70s models due to poor original quality from the supplier of the rubber/neoprene moldings, only making itself known years later. The idlers in their 50s decks (like yours) through to late 60s models are usually still fine, and are often sold on eBay to buyers looking to fix this on the later decks.
In my experience the
only thing that shifts this is acetone - and quite a lot of it - but you have to be very careful to keep it away from paintwork and the wiring insulation.
Example of eBay sale of Ferrograph idlers (I have no connection with the seller):
https://www.ebay.co.uk/itm/3-x-FERRO...kAAOSwonBZ~yrf