What fun you will have re-discovering those recordings.
Checking if flat on glass is a good idea. If not flat, a Shure cartridge with a damped brush would be a good idea.
My favorite elliptical with damped brush is M97xe
Other aftermarket stylus replacements that have brushes are not damped, like my Jico replacement for my Shure V15Vxmr. Great stylus, the brush is good, but not as good for warps as damped.
btw, you can play these cartridges with the brush down/working or up, they simply click in up position.
My other thought is that some paper dust might be pressed in the grooves more aggressively than vertical, i.e. not easily dislodged.
My old LP's sound significantly better after a good manual scrub. I do batches of 10 at a time, the brush is a baby scalp brush from Amazon