The OLD type generally refers to the Double Ring, Single Ring, Ball End, and ABC.
Somewhere around late '20 or '21 they began using a thicker top cap and baseplate, some sources refer to them as '21~'28, some say '21~'29, on the forums most just say '20's OLD type.
The ABC razors had unusual baseplates at first, with the gullets between the teeth having rounded bottoms, at some point they became the same as the Ball End baseplate.
There were some special razors like the Bulldog, Big Fellow, Gorham sets sterling silver and "solid" gold, and a bunch of different handles like the Milady Decolletee, bell tube, four piece razors with a take apart handle, and more.
But the average razor that you are likely to find on ebay and elsewhere will most likely be the thick cap '21~'28, they made them by the millions, and they are probably the best shavers of them all, but I like the thin caps too.
They are all OC razors, and they were machined, not stamped.