Hello David,
the Master decides the sequence numbers to be used in the database descriptor packets that are different from LSA sequence numbers.
if the new router is the Master on the Exstart phase:
it decides the initial sequence numbers
it is the first to sends out its own DDs listing the LSA headers it knows.
This ends in a single packet listing its own router LSA.(external routes could be locally generated too).
The DR acknowledges the DD and puts in its list of requests the new LSAs.
However, the slave / DR node has to be allowed to send its own DDs because its LSDB is bigger for sure.
probably this could be signalled with the M more bit set indicating that it has to tell something to the otherside.
I agree that RFC2328 is not clear in this part.
A test can easily be done using
debug ip ospf adjacency on both routers the complete exchange could be seen.
I mean in real world the exchange happens correctly.
To be noted that because the new router cannot preempt the DR role even if it has the highest priority often in real world the DR is the device with the highest uptime on the LAN segment.
Hope to help
Giuseppe