non transferable registration & engine no.

[Not Anumber]

My V5 has arrived so i can sort out some road tax and track down the previous owner.

Do the words 'non transferable registration mark' on a V5 mean that I won't be able to transfer a private plate onto the car or just that i cant transfer that existing reg number onto any other vehicle ?

(It's an age related plate, not a Q.)

I was surprised to see the engine number listed as 'not known' as I'd assumed it was essential. It's the first time ive seen a V5 for the car but I'm presuming it must have been without an engine number since the car was built back in 2004.
Would it provde any advantage to get this updated with the engine number now ?