The original Jelco HS-25
turned out to be quite different from its OEM version, the Sumiko HS-12
. The examples I have are separated by a couple of decades: Sumiko was made in the 1990s, and the Jelco is recent. Perhaps Jelco switched to a different alloy composition?
Today's findings are one more reason to stop "listening with one's eyes". Very similar, even visually identical, devices can have different measured performance. I wouldn't be surprised if they sound quite different too.