Blob Of Lead Adhered To Stamp

As I was looking through some one cent green Franklin stamps, I came across this one with what appears to be a blob of lead (or some other metal) on it.

I thought it was something superficial and soaked the stamp to see if it would come off. It didn't.

Could it be something that got on the stamp during the printing process?

Those stamps was print on Kevlar.

Ha Ha, area66.

Monnaie - Are you sure it's metal? It could be a plastic or hardened bonding agent that dripped and adhered to the stamp.

No, I'm not sure. I was able to break it into bits with my fingernail, and it still seems more like a very soft metal than anything else. Here's a photo of it with some of the bits beside it. I notice that parts of the design are missing on the portion where the substance was, but I don't see damage indicating that the print came off with it.

I'm new to the stamp world, but it seems to me that there are also a number of printing errors on the stamp (which I believe is Scott 632 if I measured correctly); e.g., there is a gap in the top of the E in UNITED, and the designs in and around the curlicues above the 1's are noticeably different.

My guess would be that it is a blob of some household material like putty, caulk, or paint.

Thanks. I thought that maybe this was something that has been known to happen during the printing process, but that doesn't seem to be the case!