Nice find,
@starbuck.
According to Alfred, proprietor of smetonearms.com, all SME 3009/3012 up to and including Series III included as standard the 3g AS weight (also included in the original Bias Adjuster Kit).
The original brochure has specifications and dimensions listed that are absent from the user manual, however, tonearm specs of this period rarely included compatible cartridge weight ranges.
I suspect that it will be down to the short-lived fixed end cap, to which the way rod and rider weight are attached, that will be the ultimate limiting factor here. My guess is that this was changed to being movable with Series II in order to accommodate a wider range of cartridge weights (before becoming 1 piece with the CW). Regardless, I imagine there to be sufficient room within the bakelite headshell for an additional mass plate, if necessary, even if one need be fabricated and/or drilled out in order to fit amongst the fastener pillars.
From the first SME 3009 ad...
"Each arm is complete with shell fitted with detachable pillars and fixing screws at standard 1/2" spacing. Illustration shows a Tannoy stereo cartridge in use."
Said Tannoy cartridge was a Vari-Twin, not sure how much these weighed, as cartridge makers tended not to mention this either back then.
EDIT: Seems the Vari-Twin weighed circa 12.5g!