I believe you received data from Rockwell superficial testing.  There are three types, 15T, 30T, and 45T.  They all use a 1/16 inch diameter ball (just like the Rockwell B test), but they use different forces (15, 30, and 45 kgf rather than 100 kgf).

It is not generally acceptable practice to substitute one hardness test for another without prior agreement from the customer.  The conversions are usually accurate for steels, but less so for other materials.  The real concern is that smaller volumes are involved with superficial tests, so it is possible that hardness contributions from multiphase materials, and thus overall hardness, may be variable with respect to applied force.

Good information is available in the ASM Handbook Volume 8, Mechanical Testing and Evaluation available at:

www.asminternational.org

CoryPad:
I appreciate the information, and you have cleared things up a bit for me.  The supplier has used 30T, and now I understand what he did.  I have a few more questions:
1) Is 30T acceptable for 1.8mm cold rolled strip steel?
2) Is the 'T' measurement in effect the 'raw' data for the Rockwell B?  
3) He converts it to Rockwell B - is this an exact conversion (like inches to cm), or an empirical conversion (like Rockwell to Vickers)?
Again, thanks for the response.

The T measurement was the raw data.

The conversion is empirical, just like Rockwell to Vickers.

A Rockwell 30T measurement should be acceptable for 1.8 mm thick strip steel.