The difference betweem RFM12S and RFM12B
The RFM12 appears to use this Silicon Labs chip Si4420. https://www.silabs.com/Support%20Documents/TechnicalDocs/Si4420.pdf

The board supplier, HopeRF , does not report any S version.
http://www.hoperf.com/rf/module/fsk/  

Only RFM12B and ...BP version.