The following document outlines the genotyping strategies for detecting tm1a conversion to the b, c and d forms

After treatment with a source of Cre recombinase to excise the critical exon and neo cassette (in promoter-driven lines),  the Tm1a allele is converted to the Tm1b form. After treatment with a source of Flp recombinase to excise the selection cassette,  the Tm1a allele is converted to the Tm1c form.

Please see the PDF attached for more information.