These large numbers are batch/shop numbers. I am pretty sure that they were used similarly to how Brookfield put numbers on the domes of their insulators. But the question that still bugs me is why didn't they put numbers on ALL of their molds? They only used 4,5, 7 and 8. Most pieces have no dome numbers. B-F put batch/shop numbers on most of their insulators to track daily production to pay their workers.