The question is: is there a number for the registration and a umber for the log entry?
You have a unique number associated with a registration, the registration number.
You also entries in a log. Are the logs also numbered (as in a ledger book)?
You are using a number in your document header as an index (to be filled in).
If you want the reference number itself (the same number written in the log), you can simply use
this would be understood to be the same as Log Registration No. (Log's Registration No.)
If you want the log entry, then it should simply be
log entry #
This is probably what you want since you already have the registration number in the header (somewhere?) since this is already known at the time the form is filled in, you probably want a cross reference between the registration number and the log entry number in the header (easier to look things up), this would be referred to as
Registration's Log No.
Log No. for Registration
this is because the log number is a property of the Registration and usually not the other way around since one would first lookup Registration and then look for the associated log entries.
This is called a one-to-many (one or more) in database lingo, since a Registration may have multiple log entries.