Either way I don't think they're manufactured anymore as that line is pedals was discontinued ages ago. Beyond paying extortionate prices for the few rare parts that are left (i.e. on eBay) I'd just look at new pedals if Shimano can't advise on a compatible part. Sorry!
So, Performance Bike sells them for $1.50 each plus shipping, a lot cheaper than Ebay or Amazon. They are interchangeable with other pedals. Took a while to determine if they would work but they are the same for PD6610, 5600, R540. I'm in the process of refurbishing my pedals, they getting old but still work fine.