This is vintage ACME made in ENGLAND bakerlite. Really small so I think it’s 1/4 but check dimensions that you require it may suit bigger or smaller? Centrally mounted type I assume and is symmetrical design. The Bakelite part is 70mm across and 30mm in width It opens out to max 48mm and goes down to 36mm I have synchronised the threads and they must be turned in unison. Threads work well. The cork on them is in usable condition. Some fool didn’t specify the dimensions in their listing. And another fool didn’t check ! Please check photos carefully
