[S70 B5254T]

The 91-93 cars (and some older stock '93 cars registered in early '94) had the taller, flatter headlamp unit with a single H4 bulb; they also had squarer bumpers.
The '94 850 was the first with the slimmer looking and shorter in height headlamp which used dual H1 bulbs.
There was a short period where the 850 saloon had the new headlamps, the newer curvier bumpers, but with the non-tinted and flat rear lamps (as is mine); the smoked and slightly curvier rear lamps followed for the 95 model year (generally M-reg onwards).

Looking at the cars head-on, it really is very easy to see whether you have a single reflector headlamp unit (H4) or a dual one (H1) - the pattern of the glass lens is different in front of the dipped and main beam reflectors.

Google "1992 Volvo 850" and amongst all the erroneous pics of newer ones, you'll see some with noticeable squarer faces and bigger headlamps (the original H4 type).