It would still be too modern for 1975. IIRC most winmodems in laptops dropped a bunch of ancient standards. If you can find the spec sheet it will probably show

It would still be too modern for 1975. IIRC most winmodems in laptops dropped a bunch of ancient standards. If you can find the spec sheet it will probably show 9600bps as the lowest negotiable rate. In '75, 300bps was probably the max. You'd have better luck using the soundcard than the RJ11 jack.