I was originally struggling, trying to get the train sat on the back plate but Jersey Mo linked a few pics of his rebuild and it clicked for me. By building it up on the front plate, dial side down in the clamp, you can line the train up first, then the escape and the pallet fork rests on the balance which is just sat in place, it also helps if you loosen the balance screw a bit, it adds stability to the pallet while trying to get the smaller back plate to line up with the pivots.
I then just eyeball the back plate with the keyless works and mainspring barrel and just nudge it around with an old oiler or very gently with the tip of my finger until it sits in place.
Then loosen off the clamp, slide the tweezers in (get a reasonable grip on the whole movement) and flip it over to screw the plates in.
Takes a little while to get the knack but after a bit of practice you get used to it.
I bought a load of old Accurist ETA movements on eBay for practice (all reported not working in listing) 4 of them are ETA 2390 with small variations (different balance settings).
These arrived yesterday morning. A couple of them run at least for a few minutes, but I decided to tackle the 'worst' one first, I figured it would be easier to see if I had made a difference. (It wouldn't 'start' at all despite being fully wound).
It had a fair bit of corrosion and was pretty dirty. Photos show a summary of strip down and cleaning. I manually cleaned in a small jar with horosolve, removing corrosion as best I could with pegwood and fibreglass pen, the end result is far from perfect but a massive improvement on how it started, and joy of joys, on reinstallation of the balance it started running! I got quite emotional!
I did a reasonably thorough job, relubricating as I went through reassembly, only 'shortcuts' were that I removed the barrel lid and checked spring for damage, but didn't remove it, and I didn't tackle disassembly of the balance as it seemed too much too soon (really should since this doesn't have a shock setting so balance disassembly is required to clean and lubricate top setting) I gave it a brief go, but after failing to get at the tiny screw on the stud after several attempts, I had fears of ever being able to get it back together if I did manage to get it apart.
Anyway, on timegrapher the initial readings on a full wind were amplitude of around 240 and beat error of 1.5ms, rate anywhere between -3 and +20 depending on position, so not brilliant, but I'm happy enough given it is a scrap movement that didn't run at all yesterday. I cannibalised one of the other 2390s from the lot for its hands (this one was missing a minute hand) and reinstalled the dial so I could pop it on my desk under a glass to let it run for a couple of days and check its rundown and timekeeping over a longer period. With the experience gained on this one, hopefully I can get the 'nicer' ones from the lot working a bit better!
Ok, I got it to work. It was together correctly. HSLs pictures helped. I didn't realize there was a third position. It just requires an alarming amount of pull to get it into that position.
Another came in over lunch!
This one is even running! A. Schild movement. Gold case with some damage, but the damage suggests it's at least not plated. Dial is in good shape. Not a bad score for €3,50!