the Garrard 301 plater is concaved.

My early grease bearing 301 has a slightly concave platter too. I believe it is common. I believe it was made this way intentionally. If it bothers you, there are after-market platters on the market that offer slight benefits that may or may not be audible depending on the rest of your deck and system. Check out Peak HiFi in the UK. I bought Shaun's replacement brass bearing even though my original bearing looked brand new. I am a big believer in having the stoutest most precise main bearing possible. https://www.peakhifi.co.uk/cgi-bin/ecom.cgi?Command=ShowProduct&db_pid=718
https://www.peakhifi.co.uk/cgi-bin/ecom.cgi?Command=ShowProduct&db_pid=96

After noting the comments of the "slightly concave platter"
I proceeded to check my mid run 301 grease bearing 
and low and behold it is slightly concave.

There's always something to learn in this hobby. 

I believe it was made this way intentionally

I don't know if it was intentional, but I would think it's easier to make it flat, certainly if turned on a lathe. A slightly concave platter, used with a clamp, helps flatten the LP. 

The original platter was not turned on a lathe. It was cast and then spun balanced with drilling of the heavier spots. I wish the chassis was also cast-but it was stamped. There is lots of literature available on this. EW Mortimer's book on the history of Garrard is a good start. 

everything I ready prior to my posting said it was a design feature, I was just surprised to find it in a table designed in the 50's.  The old Garrard's were better designed then I originally thought, well at least included some modern thinking.