Seems to be a reference. I would not attempt to "translate" this

I think the M stands for manual and this refers to the number under the old manual registration system before the issue of the NADRA certificate.

The following site includes a sample image of a NADRA birth certificate:

Procedure to Get Pakistani NADRA Birth Certificate
...
Pakistani computerized NADRA birth certificate can be issued from following authorities depending upon the address and area where you live:
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Requirements to obtain NADRA birth certificate:
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8. Copy of child’s old (manual) birth certificate (if applicable)
http://www.translation.pk/language/578/procedure-to-get-paki...

What is NADRA Birth Certificate?
NADRA birth certificates are modern and much more authentic form of old manual birth certificates. These are fully computerized birth certificates and contain complete birth record of a person, both in Urdu and English language in one document.
http://birthcertificatespakistan.com/tag/nadra-birth-certifi...

What is difference between NADRA Birth Certificate and Manual Birth Certificate
About NADRA Birth Certificates
NADRA birth certificates are fully computerized and authentic documents printed on special NADRA issued papers. NADRA Birth Certificates contain both English and Urdu contents and there is no need for English translation. NADRA Birth Certificates are linked to NADRA database.
About Old Manual Birth Certificates
Old birth certificates were not computerized documents if we compare them to NADRA birth certificates. Manual birth certificates contain any one language like Urdu or English.
http://birthcertificatespakistan.com/what-is-difference-betw...

The following discussion thread also includes a redacted image of a certificate including one of these numbers:

Does he have a pre-NADRA birth certificate? Your scan shows that there is an "OLD/M REG NO.", which means there was a[n] old/manual/pre-NADRA birth certificate. That could also be helpful.
https://www.visajourney.com/forums/topic/746435-we-just-got-...