Serial number on magazine not matching is really not a problem.
The Mk.1/3 designation means it went through an FTR (factory Thorough Repair) and was upgraded to the No.4 Mk2 configuration.
I have seen asking prices all over the map for No.4 rifles but have seldom seen any actually sell for over $500 usd.

---------- Post added at 01:21 PM ---------- Previous post was at 01:19 PM ----------

I would like to know what book they have that says an Enfield No.4 Mk.1/3 is worth $1000.....