Yeah, apparently the only difference between the earliest vented T75's and G65's is the former material; kapton or glass fiber.
@DanTravis62 is your 1965 a slanted or straight cab? My converted one (1965B-->1966B) is the straight one and it's definitely the fuller sounding of the two, even after testing both with vintage T75's each.
Sound/weight ratio they're really hard to beat and when I'm speaker swapping for tests and with my face on speaker height, the 1966B sounds so awesome for recording too.
I have another 2x12 cab; some Polish company custom made it, I bought it used and empty and this one has just slightly bigger dimensions than the 1966A/B cabs and weighs probably 20-30% more; this one is about 25" x 25" and I got some dense (40kg/m3) absorption material for it and lines all the insides AND the backplate(s).
This cab now (with a G12-65 and vented T75 combo) does the big, gut-punching Recto-thing even more than my 4x12. It's borderline too much and I'd probably piss off most bass players (and my neighbours already
) with this.
I'm still hoping to find a used, empty, broken grillcloth 1966A or B, so I can put some checkerboard grill cloth on it and throw in a pair of my pre Rola G12-H30 55Hz for vintage Greenbacky goodness. This custom cab has been done too nice to remove that grill cloth...