The ES37 is an 8-channel Quantizer. 4 of the channels are presented on the front panel for user interfacing.
The other 4-channels are for internally patching to units such as the ES38 to provide quantized outputs in addition to the original outputs. The ES37-Expander is added to gain access to the extra 4 quantized outputs.

www.elby-designs.com/webtek/euro-serge/es27-quantizer/es37.htm