plumbing
I'm trying to connect my new electric shower unit up. Unfortunately both the connections have threaded connections. I bought a straighter coupler but this seems to be the opposite of what I want.
This is the existing elbow joint.
This is my failed attempt to bridge them with a straight coupler. Posted to give an idea of what I'm trying to achieve. 
Sorry, this is probably the stupidest question ever. I'm new to plumbing.
I would call it a slip x spigot brass drain street ell. Can't say I've seen one until now. More common is the double slip brass drain ell. Hard to say from a photo, but looks like a short horizontal straight piece could be inserted if you had to use a double slip ell. Perhaps the trap U could be rotated away a bit to buy a little extra distance for this.
It looks fairly standard to me. I'd contact Eljer & see if Danco, Brasscraft or someone else has put out a replacement that fits. Otherwise, have you tried to rebuild the old valve with O-ring replacements & such? Eljer should be able to direct you to the right kit for the valve.
On the other hand, soldering will be difficult & could cause a fire in the wall if you're not experienced. So, either get a Plumber or get a SharkBite Threaded connector so you can just cut the old valve loose & wrench in the new one. I updated the picture to show the correct valve that has screw shut-offs or stops (the silver screw).
Best Answer
So I ended up using Copper Pipe 15mm. Used a 15mm pipe cutting tool to make a tiny piece. Then I used 2 15mm nuts / 15mm olives on either end and managed to bridge the gap.
I also added a 15mm isolation value so I can work on the shower in the future without having to isolate the entire house from the mains.