The few NIDs I've been inside of had the serial number stamped in the head of the stock.
It appears that forearm was highly modified to move the Deeley latch quite a ways forward. A Normal No. 5E forearm --
No. 5E forearm outside.jpg
The OP's forearm --
Odd No. 5 02a.jpg
It appears the base of the forearm iron is an ejector iron with the metal yoke that the screw at the back of the forearm wood goes into.
Odd No. 5 01.jpg
447333 05.jpg
A normal extractor forearm --
443760 02.jpg