You're right - these are pretty close but do have some major differences:nymphaeles said:
The 7518 and 7519 are fixed base 3.25 hp routers
7519 is a single speed router
7518 is a variable speed router with 5 selectable speeds
The 7538 and 7539 are plunge base 3.25 hp routers
7538 is a single speed router
7539 is a variable speed router with 5 selectable speeds
The 7519 and the 7538 will need to utilize an external speed control device if you are going to use the large router bits for raised panels as opposed to the built in internal 5 position speed control on the 7518 and 7539.
I hope this helps clear up the confusion.
Bill