Company forcing to pay two months salary in lieu of notice

After working in ONGC for 8 years, I resigned on a month's notice (instead of 03 months as standard). I have now been asked to deposit (02 months (unserved period) + 03 months (Penalty)) basic + da. I have gathered that such a random penalty of 03 months does not exist anywhere including private companies. I have two questions: 1. Is it legal for a PSU firm like ONGC to impose this penalty. To impose this penalty, they used an office order signed by a GM (HR), issued in 2012 citing amendments in service rules para. 2. Interestingly, There is a copy of service rules published in 2014 (It is unsigned but available on company's employee portal for two years and provided by officers). The updated service rules copy does not have any mention of 03 months penalty. On inquiry, HR says that they have forgotten to update the same hence the amendment issued in the past would be valid. Shouldn't the updated copy be assumed valid in such a scenario?