According to
http://www.spectrumaudio.de/breit/vintage.htmlFostex UP203 20cm*8Ohm*40Watt*SPL91dB*Fres40-20kHz*Qts0,55*18,5g
Fostex UP203sigma 20cm*8Ohm*40Watt*SPL93dB*Fres40-20kHz*Qts0,45*18,5g
I have never heard of "Fostex UP203sigma". I guess they mistook it for "Fostex UP203super".
Fostex international web site lists their products compatibility and most of their previous products, but no entry for Fostex UP203sigma.