Unknown bike brake system component

I am currently in the process of restoring a vintage Tsunoda Grand Prix bike, and have replaced the caliper brakes with new ones, however there is a part on the brakes which is made of plastic and I would like to replace, but I cannot figure out what it is called. It sandwiches the caliper spring in between itself and the calipers, any help would be greatly appreciated. Unfortunately I cannot post a picture of the part as this is my first post on here.

Thank you, Ben