A/C receiver drier leaking

['79 Super Turtle]

I have three W116's. My 1980 450 SEL has been leaking from the receiver drier. There is a threaded bolt on the top that appears to be original. It has a splotch of white paint applied to where the bolt meets the receiver drier. This appears to be a plug. The very center of this bolt/plug has a small cylindrical plastic center that 'rose' up and started leaking. I sort of pounded on it and it smashed down like plastic. The other two cars all seem to have a replacement brass plug inserted into the top of the receiver drier.

Is this a common leak? Has anyone ever replaced that plug? I assume I need to have it evacuated, replace the threaded plug with a new one and refill?

I'd like to take the car to my A/C guy with the new threaded bolt to switch out. If you know what I'm talking about please reply. Thanks!