http://www.potteryhistories.com/midwinterhistory.html

This gives the company history,, If you go to the marks link at the top,, you will see that the marks on your pieces are showing that that it was possibly made sometime around 1946 !! The circular mark was used from 1910 to 1946 and the crown mark was used from 1946 to 1953 !! So if your set was made around the 1946 date you would find either mark !! All pieces of a set were not made at the same time !! There can be a time difference between when the pieces were in production !! Cups may be first and saucers may be next !! Remember these are molded products so may have been several production lines !!
And not all pieces of a set were necessarily purchased at the same time !! Sometimes one may purchase basic pieces and others were added to the set as one could afford them !!
Depending on how well a product sold,, they may also have several runs on the pattern within a several year time span !!
Value on most of these items is minimal !! If yours gives the pattern name you can look on ebay to see what the sold prices are !! Not all will have a pattern name some will have a shape name or number only !!