The last time I requested a backup disk was when 3.5 first came out and that was more than 5 months ago I think. I did get a 3.5 disk back then but that was before r2. Today I got another ZBrush 3.5 disk in the mail with a totally new CD artwork.
Is this a back up disk of Release 3?
Because the installer is simply labelled ZBrush 3.5_intaller and I don’t want to have to start the installer just to find out.
The disk is 3.5R3.
Earlier releases of 3.5 were a zip file; not an installer. So the fact that your disk has an installer means it’s R3. 
We held off on having Cleverbridge send out most of the backup CD’s because we wanted to ensure that 3.5 was finalized in order to give you the best value for your money. For a few users who upgraded right when 3.5 released CD’s with 3.5 went out. But now that R3 has finalized version 3.5 we are having any such CD’s resent with the newer version. That’s why you got a second CD today.
Nobody is being charged for this second CD. It is something that we’re doing because we didn’t feel it was fair that you received a CD at the beginning of 3.5’s life that was basically invalidated as soon as the next update came out.
Thank you Pixologic! 
As always, you guys are awesome!