I've put in requests for quotes to a number of companies now for 100ppm coatings.  I also checked through our history of measured transmissions, and notices that these mirrors have some that are very close to what we want.  The Y145P 2" mirrors that I previously measured seem to have much higher transmission for 45S than the ones in the Mott measurements.  I'm going to go across and dig these out again so we can repeat the measurement.