Affy SNP-chip metadata - how to read the probe sequences?

I have Affymetrix SNP-chip (250 K genomic) data. For each gene there are 56 probes =7 "quartets" (windows)*AB*PMMM*SenseAS. The information in the accompagnying CDF/TXT file looks like

SNP_A-1780520 rs16994928 ggatagtgttgacctc[A/G]agtacaggtttcaaaa 496 // 47873735 // 47874230 0.0 0.11 0.0

ProbeSetID Chromosome Expr1002 PhysicalPosition AlleleA dbSNPRSID

SNP_A-1780520 20 47874178 47874178 A rs16994928

I need to intepret these meta-data in terms of the probe sequences. In particular, where in the ggatagtgttgacctc[A/G]agtacaggtttcaaaa string are the seven quartets and how does the MM probe difer from PM probe