There are 3 different headstock casting versions.
"A"is the original sand cast headstock. It is rougher and thicker(heavier) and does not have an access hole on the reverse side.
"B" is the most common version. It is diecast and added the access hole on the back side(in the area of the two small holes on yours).
"C" is the current version and the access hole was enlarged and the power switch changed and moved.
There are three different power train versions.
The early Mark 5 had a Gilmer(cogged belt) drive and a 6 spline quill shaft and a 3/4 hp motor.
The later Mark 5 increased the motor to 1 1/8 hp and changed the drive to a poly-v belt and the quill shaft splines were increased.
The Mark V is essentially the same as a Mark 5, but has four different table configurations. 500,505,510,520.
The current Power Pro radically changed the drive train and includes a stronger DVR motor and simplified drive belts.
Color wise, there are the Mark 5 Greenies(like yours (sorta)). Color scheme silver and green hammertone.
Then came the early Goldies which were essentially the same as the Greenies. The Goldies are sometimes referred to as Brownies. Color scheme copper and light brown hammertone.
Next came the Goldie(Anniversary model(#1)) which included the 1 1/8 motor and poly-v drive.
Then came the gray wrinkle paint.
Then they went out of production.
When production resumed, they were again Mark 5s and gray. At some point in time the gray paint was changed to a different texture
(I call it splattered) and is current.
More re the Mark V sub models.
The 500 is the same as a Mark 5.
The 510 added the larger main table, floating tables with round rails, adjustable legs for additional support/set up ability.
The 505 was a stripped down 510 that included only the larger tables with round rails.
The 520 changed the round rails to the current boxey type(a notable improvement).
The rip fence design changed each time the tables changed.
The anniversary model(#2) has a different access hole cover.
The Mark 7 added tilt both direction as well as the Power Pro headstock.
Since even the earliest Greenies can be upgraded all the way up to a Mark 7 it is not always easy to discern what one has.
There have been less major design changes, but these are the major ones.
It is important to know just what one has if parts are needed(especially if procuring used parts on e-bay etc.). Some out of production parts are only obtained used.
I hope this does not overwhelm y'all and serves as an expanded 'glossary'.
Keep in mind there have been several corporate entities that have produced these models over the past 3 score +7 years. That so much is the same/compatible/upgradable is quite unique and one reason for the fierce loyalty of it's older owners. And another reason many of us have more than one.
FWIW there have been other models - Mark 1(only two known of), Mark 2 and Mark VII - as well as a few 'clones'.