Constabulary

machine seems to be a Singer 17 or Seiko TE-6 and they have supper small bobbin. Surprises me that they sew with fairly think thread this fairly thick leather with a machine like this > not much thread on the bobbin. When I look at the stitch length lever the it seems to me just a screw w/o reverse - or not? However - IIRC there was a reverse device of the Singer Class 7 that did not come with reverse ex factory... Maybe they tinkered something similar. Well.... lots of guessing here. No matter what they do a great job there - I´m quite impressed. EDIT: weeeeell - I´m not sure - new TE-6 have reverse AFAIK but not sure what I see there. EDIT II: Looks like this one - which looks like a non reverse machine

thread is coming of the spool a bit the odd way. Maybe one of these thingies may be helpful. However if it works for you then don´t mind. 😉

okay, may depend on the machine 😉

I´m updating here cause I have an Astor M5 on my bench for restoration - the inner thread is metric M10x1.5 (at least on mine). So the information MikeRock got from the distributor is correct. So if you need a cheap tool adapter for M6 or M8 tools / dies then this could be an option: https://www.ebay.de/itm/402466257921 Hope this helps

not sure what you mean... Is it that your last stitch is probably to long and not hitting the 1st needle hole where the seam began? If so then you can shorten the stitch with the reverse lever and hand crank the last stitch - if you know what I mean.

not sure where how you figured that but I have different experience (I´m living in a metric country). Maybe someone overcut the original threads on your machines. Nothing really metric I´d say: https://www.singersewinginfo.co.uk/technical/parts/screw_threads/

Or NTB in the City where the Adlers came from: https://naehtechnik.eu/

Check with Liersch: https://liersch.com/online-shop/ersatzteile/ or Zieger: https://www.np-z.com/ or Busche: https://busche-online.de/ If they don´t have it in the Shop send them an Email - Answers may take longer due to Christmas season.

I never did that drop test, the manuals show how to set the tension right. That works for decades, just follow it. But as always - many roads lead to Rome - or each to his own. 😉

BTW - I put one off these in a plastic box and wired it to the motor. https://www.ebay.de/itm/396873227544 But I´m using it for my electric bobbin winder which run with domestic sewing machine motors. Not sure if this is a solution for you. They are available everywhere and I think even cheaper. This was my project back in 2017

servo motor and speed reducer come to my mind but depends on your sewing machine table and if the machines have a hand wheel with pulley and no built in motor.

these original manuals pop up every now and then. i just recently bought two from a guy in France he still has some: https://www.ebay.de/sch/i.html?ssPageName=&_ssn=zougoulouba

Guessing - most likely a motor problem. I guess the machine has a servo motor so I would guess the lever of the accelerator unit or the hall sensor is loose / has moved and is probably stuck in the "run" position. Please post some pictures of your motor and the accelerator unit and lever where the it man rod is attached and maybe a video showing the issue (please not just 5 second video or so" upload it to YT and post a link here.

yeah - keep that old cast iron alive - I seldom went wrong with that! 😉 I love my 51w but the 2nd type as yours would be even more preferred but I love mine too. Singer crazy me is. 😁

often Ebay is a bit cheaper than Amazon - even from the same seller. Just saying...