I have wondering as well. I have a thought it was some type of stencil or overlay.
There aren’t any videos that I could find specific to that hubcap.
Here is one on a different type of 67-72 Chevy that may generate some ideas or how not to’s.
https://youtu.be/yApxN6hMF-M?feature=shared
I have also looked to emblem restoration for tips. One such tip was to use a syringe for filling computer ink cartridges and model enamel paint.
DIY Emblem Restoration
https://youtu.be/rCJOgaaqAlU?si=wm4VrNqwdZ-jwzFC
and a small paint brush technique on a grill center piece that would've worked with the syringe method.
https://youtu.be/lbXglhM079M?si=FfMFKbtWGK_DeK58
I have three that are in fair to good condition, 62 GMC Painted caps, to work on.
Looks like there is enough of a "well" to put the red in there.