Are there any additional accessories or a manual in the drawers?
It sure looks like a "badged" machine. I'm not familiar with the Mason/White connection. I found
http://needlebar.org/cm/displayimage..._display_media that looks like it may be like your machine and "Arnold" is a badged name for A.G.Mason sewing machines.
There is a 'sticky' about cleaning these old machines at
How to Clean Up and use a vintage sewing machine - videos by Muv and Fav Wiping with sewing machine oil on a cotton ball, changing cotton balls frequently so you don't scrub dirt and scratch the decals.
It looks like it has a needle thread tension release like the early Singer 27 machines. One needs to press it to release the tension on then top thread (after raising the presser foot) so you can withdraw the fabric that you have just sewn.
If you want to disassemble it to clean and polish the metal bits, take lots of pictures during removal.
From my research, it looks like it may take what is called a 20x1 needle. I've read that one can drop a 15x1 needle a bit and it will work. One person put a piece of sheet magnet in the needle bar at the proper place to make it easier to use the 15x1 needle. I had a machine that supposedly took the 20x1 needle but when I was cleaning it up, I found several broken needle pieces. I tried the 15x1 and it worked fine. Evidently, someone in its prior life had adjusted it so that it would work with needles that were more readily available.
Have fun getting her back in working order.
Janey - Neat people never make the exciting discoveries I do.