As I recall there are two different lengths depending on which 42C mower you have. 42C-4 and below take one length and 42C-6 takes a different length. Best to consult Rudi's manuals to make sure you have the correct deck ID then check the parts section in that manual for the correct belt part number.
IF it's a 42C-4 then from my "belts and bearings" spreadsheet I keep handy;
Drive Belt 5/8 x 136 Woods 1469 PIX B133K Gates B133
I don't have a "6" so I don't have it listed.