Fairly common in uprights and some grands, I've seen these used in the regulating rail as well in the wippens (usually as a component used for adjustment on the repetition lever). 3/8" diameter, 1/4" thick.  Related parts such as regulating screws and felt punchings are available in several variations in my eBay store.

I've been a piano & player piano technician for 35+ years,  I'm glad to answer any questions about this item before or after the sale - my contact info is included with each shipment and you can also reach me through eBay.