The Ford motor foundry was the source of hundreds of nice Hemingray blue mice, usualy with the Made in USA added to the 1893 types. The stories from the old employees, with slight variations, usualy go "we got hundreds of those off of the roof". Some of the enterprising finders heard that it increased the insulators value if they were crackled. So they dipped them in boiling oil. The insulators werent crackled but they were filled with many large fractures. They were to be found in antique shops in the area for many years. If you have a blue mouse with large fractures filling it, this could be the source.