Four examples of type 4 mintmarks; 3 (above) with type 4.1 have star limbs straddling the top of the crown ornament, and one example, right, of 4.2 where one star limb points vertically downwards.
Wedge-shaped R
Pointed chin
Mint mark 4
Note - the terminology 3d1/3d2 was introduced by Churchill & Thomas :- these coins were previously included in class 3c.
Ball footed R
Pointed chin
Mint mark 4.2
Note - similar coins but with an eight-rayed mintmark are class 3d3.
Cruder bust compared to 3d1
Wedge-shaped R
Rounded chin
Mint mark 4
Wedge-shaped R
Mostly mint mark 4.1, or rarely with a variety of mm5, an eight limbed mintmark.
Can have the bust of class 3b or a narrower bust, transitional between 3b & 3c.
Ball shaped R : Class 3d2
Wedge-shaped R: 3bc, 3c, or 3d1
Usually mintmark type 4, star with blunt ends: rounded or angular.
Also note that coins from these classes often have a colon after REX.
Some scarce 3bc pennies have an eight limbed mintmark -though 8-limbed mintmark coins can also be class 3d3.
To differentiate classes, firstly look at the shape of the letter R....