I'm not involved as I shoot neither Mamiya nor Schneider, but I can mention some thoughts about the issue anyway.
To begin with, I don't think lenses care about where they are made, nor do they care about where the glass they use is made.
Lenses care about the way they were designed and assembled and the way they are coated.
A bit related, but I tried to check out some of my lenses for Zeissness. I have set up a session with 5 different lenses around 70 mm on a Sony Alpha 77 SLT (24 MP APS-C camera).
The lenses were:
Minolta 80-200/2.8 APO
Sony 28-70/2.8 ZA (having Zeiss label, T* coating and Zeiss serial number and with a QC certificate signed by a Zeiss employee)
Sony 16-80/3.5-4.5ZA (having Zeiss label, T* coating and Zeiss serial number and with a QC certificate signed by a Zeiss employee)
Test subject was a pelargon and a small USAF test target. Also included was a slanted edge and a mini color checker. As far as I can recall f/5.6 was used for all and focus was in live view at highest magnification.
Once the five images were matched so white balance was set on the second brightest gray CC patch and exposure adjusted I was not able to tell the five images apart at actual pixels. I have not done MTF evaluation on the slanted edges yet.
This test was done near to the optical axis, so corner performance was not tested. But I wouldn't say that special Zeiss characteritics were obvious to me.
Recently discussing lenses with a couple of other photographers the German glass discussion came up.
One mentioned how he prefers the German glass such as Phase One Schneider over Japanese Glass.
I pointed out that the Phase One lenses have made in Japan and they are also made under the Mamiya Sekor name.
There is this video on the Phase One website:
With this description:
"The making of LS lenses
Phase One and Schneider Kreuznach work closely to provide high quality lenses to the demanding
professional medium format photographers. Go behind the scenes and see how the leaf shutter lenses are made"
No mention of Japan in the video...
In the video they mention Schneider buys the best glass. Hmmm who do they buy from... is their supplier German?
The Mamiya branded LS lenses are also less expensive than the Schneider/Phase One branded lenses.
In this Phase One video Adrian Weinbrecht goes on about the differences between "German glass" and Mamiya lenses.
So where's the glass from?
Why are Mamiya LS lenses less expensive?
Lenses are labeled Lens Made In Japan...... so isn't that where they are made?