How to extract the 15 digit IMEI from 21 digit IMEISV?

Dear Experts.
How to extract the 15 digit IMEI from 21 digit IMEISV?

Are you sure that IMEISV has 21 digits?

I know 16 digits:

From 3GPP 23.003 (16 digits):

This is what surprised me.
I think beyond 16 is the device defined and not part of standards.

Check this: