I think the original seal is made of lead. Replacements (even the Omega ones) were rubber or nylon. It's a sign that the watch was likely never serviced from new. Most watchmakers would have routinely replaced the seal every time the watch was serviced. Hope this helps, gatorcpa
if you still have the lead seal, re-fit it and do NOT throw that away. keep it with the watch . you can easily bend it back into shape. kind regards. achim