Difference between brake calipers....

[Neunelfer]

for my 1969T, i am converting it to 1969S brakes.

The front struts need to be changed to accept the aluminum callipers.
What would need to be changed on the rear to convert to the 69 calliper for vented rear discs?

You'll just need the 70-83 vented rear rotors, which are much less expensive than the 69 rotors and, a pair of 70-74ish 911 rear calipers. These would all have the knockback pins to match the front calipers. They're the exact same calipers that came on your 69T but, they will have the 10mm spacers and longer fasteners to accommodate the spacers. You could just get the spacers and fasteners but, you'd have to get them from a core caliper so... just get a set of calipers from that vintage and be done with it. If you're having a hard time finding the 70-74 rears, any rear caliper from 70-83 would work to steal the spacers and calipers from and install on your 69 rears.

[Neunelfer]

If the calliper has the spacers but dosnt have the sharp angle gussets underneath, what model year would they be for?

1970 and up (until the end of the M-Caliper) got a smoother surface under that nose section...

[generalsmirnov]

Although Eric is the master, I happened to have a couple of Early S calipers come in at the same time and so made a video comparing and restoring them:
https://www.youtube.com/watch?v=7VY_KvGUqlA

Any feedback welcome