Welcome, sewhopelesslyvintage.
When I first read the post, I was hoping you meant taking the cover off the base of the machine to oil. Then, I saw the picture. I'm wondering why you were even trying to separate the arm/pillar from the base. I have done a lot of research and have three featherweights and never seen a separation like that. I am under the impression that even those that repaint featherweights do not separate the pillar from the bed. I'm fairly certain when it was being made, it was in two pieces. Does it wobble when you sew with it? I have a bunch of other questions, but not sure it would make a difference.
Should you be determined to do dismantle it to that extent, I would suggest asking someone like Carmon at
https://singer-featherweight.com/pages/contact-us I'm Not affiliated with that link, I just know they have been around FWs for a long time and were able to take over that web address from Graham Forsdyke of London, England, the world renowned purveyor of Featherweights, when he retired.
Janey - Neat people never make the exciting discoveries I do.