Where did you find that information? The early Volvos had an offset of approximately 25, It is definitely not possible to fit a FWD wheel to a 140. And it should be if your information is correct. I tried it today with a 15" steel off a V70 onto a 1969 144S as a sanity check. It sits far too far in and interferes with the front suspension.

I've also in the past tried both 940 and 240 wheels on a 164 and that combo fits perfectly.

The 240 did indeed go to an ET of 20, hence there were dire warnings from Volvo at the time about not using 140 series wheels on 240s. This was, as I understand, aimed largely at people swapping winter tyres, the extra 5mm inset was critical especially if a set of chains got thrown on.

ET stands for Einpresstiefe, or insertion depth in English. Therefore a wheel with an ET of 20, as on a 240, will sit further out from the face of the hub than one with a FWD ofst of somewhere between 35 and 40. The distance measured is the distance between the mating face of the wheel and the centre line.

Note that the wheels listed for 700 and 900 series cars do vary, the 14" Scorpius is a 15ET, a Polaris has 20 and something like an Adhara is 25.

ET on its own is ofcourse no guarantee the wheel will fit, overal width and diameter also plays a role.