Series III newbie

Hi, everyone newbie to land rover ownership and already need help!
Firstly, my newly aquired 109 is fitted with a later 2.5 turbo diesel, can anyone tell me where the engine serial number will be located. Also will a 110 exhaust fit straight on as the previous owner appears to have welded a bunch of tin cans together.
Secondly, I am happy with the use of the 3 knobs for the gear box, however when trying to engage 4wd in high ratio the yellow knob will not stay down and insists on springing back up without engaging. Has anyone else experienced this or can you please help with suggestions.

I dont no about the serial number but the gerbox is simple!

if you want 4x4 highrange you push yello nob down, when you want it out of 4x4 hugh range you put it in low range (red nob) and yello will pop up. then you are in low range 4 wheel drive then if u disingage low range you are then in rear wheel drive high range!
Simple enough??!
So what i am saying is yellow nob is suppose to pop up when you pull the red one!!

hope this helps.

Thanks,

I was under the immpression that to engage 4wd whilst in high ratio you depressed yellow lever and it stayed down, to disengage you select low ratio and then back to high. My yellow lever however will not stay in 4wd in high ratio.