No Denis, it is indeed not a real IMO number - they hadn't been invented then. But it is a 7-digit number from the same sequence used by LR which was later taken up by IMO, so it works in the same way and is widely used nowadays.. Miramar take it from LR also. Call it a "proxy IMO" perhaps and, as has also been said, it works very well here to group photos.

Yes, then she is also grouped together with the other 2 pictures

Is it really her IMO but not ID number from Miramarshipindex?

IMO 5607468