The “TDR” means it is a td-18, I have never seen it printed anywhere as to what the “r” actually means . The “suffix letter code” at the end of the actual serial number will tell what options or the configuration of the machine . The suffix letter code can be found in the rear of a parts catalog .
The serial number prefixes for some of the other models are as follows :
tdbk-td6
tdcb-td9
tdf-td14
tdr-td18
tde-td24
i hope this helps