IMEI code /01 explanation

You are probably refering to the SVN - Software Version Number. It is often displayed at the end of the IMEI (which is then named IMEISV).

While, for any given phone, IMEI is constant, SVN can change after software update.

In your case /01 means software version 1.

My personal experienced showing that Imeisv 01 mean your phone hasn't been though service (or fixing). If the number change to 2 mean it's been though service once and so on.