Diff lock pipe for hydraulics on my 1993 300gd

Assume you mean the pipe from the reserviour down to the flexy pipe?

EPC lists the front as 4.75mm x 0.7mm x1590mm, rear only says 4430mm

So asuming they're both the same diameter it's 4.75mm with a 0.7 wall thickness and front being 1590mm and rear 4430mm long respectively 

That was for a 463 LWB

Actually just looked at a 463 SWB and obviously front the same but rear actuallly says 4.75mm x 0.7mm x 3980mm

So look to be same diameter and bore just 450mm different for 2400mm SWB and 2850mm LWB which would make sense of course!

I fixed that, but just do a 'Search Website' (top left) and you will find the thread.

Rust will make things bigger,  I have renewed them in the past, nothing unique unlike some of the fuel pipes

Had a quick look under my 463 earlier... a bit newer (1998) and pipes in great shape and look 3/16 standard Brake Pipe to me

Parts picture is a shockingly over simplified schematic!!

But looking the unions etc they all are standard brake pipe size

74 is the front pipe

68 is the rear pipe

Front flexy goes straight to slave cylinder but rear flexy has another section of hard pipe after flexy onto slave cylinder and thats shows 3/16" also 77 on picture

Slave cylinders are identical to 460 and that deffo has 3/16" pipes 

Obviously 463 Master set up differs to 460 being electro hydraulic rather than just all hydraulic but after that they are basically the same idea

I am 99.9% sure it's standard brake pipe and has just "blown" with corrosion

They will have been steel pipes from factory coated with a protection so will react different to cupro nickel (cunifer) pipe commonly used as a replacement

Trace the pipes to where they join at either end but probably clearer where they meet the flexy pipes and you should see the union size and hopefully find a section of pipe that hasn't corroded so badly?

Brake pipe (3/16") unions are usually 10mm or sometimes 11mm spanner