Is it possible to automatically detect index of files in a nested array

First of all I would like to wish you all a successful, prosperous but especially healthy 2021. I also would like to thank all power users on this forum for all their helpful advice given during the last year.

I have a large file that contains two types of measurements. The difference between both measurements is the length of their header (10 rows versus 11 rows). I would like to define the index of the files containing 11 rows what would allow me to easily remove the ones I do not need.

I am using MC15 but till now I was not able to achieve my goal. I am even not sure whether this is possible. If it is not possible are there other ways to achieve what I want? For illustration purposes I have added a small file that hopefully will clarify the matter.

All input is highly appreciated!