Hi dude,it should be under the battery cover,on a sticker,centre position. Mine (1720),is a 9 digit number code,with the original serial number on my first cert,a 16 digit number,prefixed with what must be the 7 digit model code.
There may well be duplicate numbers internally,but i cannot remember seeing one,the last time i dissected one...
For the purpose of a calibration certificate,it just needs identifying,for which reason,you could mark up your own.
This is NOT an endorsement of ringing a chorred piece of kit....the penalty for which,should be brigandage.:bigear: