Here are some notes on build 125 of dbSNP.

b125_ContigInfo_35_1 table describes the contigs of the assembly
   .ctg_id field seems to be primary key
   .ctg_acc is things like NT_XXX - the contigs we use normally.
b125_SNPContigLoc_35_1 table describes the contigs of the assembly
   .asn_from and .asn_to seem to be position within contig
   .phys_pos_from and .phys_pos seem to be position within chromosome
   .snp_id this is the rs number, minus leading zeroes if any.