Must be someone else, my only Guards unit was 3rd Guards
BMT in Nee Soon.
Not too familiar with the Magach, the only tanks I’m familiar
with that had M48/60 were the American Walker and
Patton. Any idea when David served? If he’s very, very old, could
it be that we got some surplus Super Shermans from the
Israelis?
As for the Centurions, I’m guessing they were either for study,
target practice or to scare others off judging from their condition
(rusted brown and hull paper thin, missing tracks, roadwheels etc)
though that could have been after they were declared scrap. Totally
unusable.
Talking about unknown tanks, I was in NS when they were
roadtesting the Bionix under dri-clad covers, my unit kept
reporting them as Stormers due to their shape. We thought the
SAF was going British for a while. 
Yup I thought I remembered that. I just thought Guards only
conduct mono for their own intake.
Magach are just Israeli-upgraded M-48/60 upgunned to 105mm.
Super Shermans as medium tanks would predate the definition of MBT,
and probably would be barely serviceable by 1970.
Btw I posted awhile back, if you go to Army Museum at Safti they
have AMX-10s with (one IFV and one 90mm) on display in what appear
to be Opfor colours (black spots). Perhaps in addition to
evaluation, the MBTs served in the Opfor role at one time.
Another possibility: David occassionally alludes to the mystical
world of official secrets once in awhile, without giving anything
away on his blog. He surely knew it would fuel speculation when he
made that cryptic statement "fast landing craft are also needed to
ferry the SAF's third MBT type, the Leopard 2SGs" and tersely said
the M60 did not count as the second. It's in keeping with his
style.
As such, these hypotheses of his may not be 100% precise, to
avoid bracketing the secret. If he indirectly lets one out, all his
access will be gone. He might have been dramatic for drama's sake,
and the vehicle was not strictly an MBT but simply a direct fire
system.
The Govt might fuel such speculation for various reason, through
its own unofficial channels or by giving selective access to people
like David.