Hoodgem and PC1, I think Borgforever has pointed this out before. I tend to agree after watching that AKAI footage.
Newcombe clearly comments while watching him that "he's increased the power of his serve"...
Here's some of that footage for reference:
(Remember he beat Lendl too Indoors, at this big money event). He's cranking some shots. Now, take out the Bancroft he was using here, and replace it with a current Wilson or perhaps Head or Babolat. What would you see then????
Supposedly, Borg worked to get stronger by 1982, in terms of upper body strength, which perhaps really helped his first serve even more by that event, though he was no longer on the Tour regularly. Although, I will say, he looks to have been serving pretty impressively even in the 1976 Wimbledon Final vs. Nastase.
I found a Youtube clip that TW Poster Borgforever has already kindly uploaded on YouTube after some reworking of the footage on his part. Borgforever, thank you sir, I hope you don't mind me posting this before you have a chance at a new thread that you mentioned.
(Part 4 only) Borg wins all 21 sets of the Tournament!
It's interesting to see how he developed physically by 1982, by comparing the two clips.
As an aside, see this clip of some "vintage" service motions: