I think it's much more a difference between the 1991 Orange shape and the 1993 shape, rather than a difference between Clockworks and Prestiges. The 1991 or earlier Prestige shown below looks the same shape as your 91 Clockwork. It has the higher top tube junction with the seat tube, and it has the shorter (11cm?) head tube. I don't think we can tell though from this photo whether it has a longer top tube than a 91 Clockwork (but my guess would be not).
To be honest, although we respect old bikes, design was moving forward, and this change of shape was an improvement not untypical of the way that mtb design was going generally around then. Also frame weights got lighter around that time, as tube manufacturers became more sophisticated in their approach and began to realise that they had initially been erring on the side of caution in specifying mtb tubes.
Incidentally, I can't tell the difference between 89, 90 and 91 Oranges. I have a photo of a Clockwork that looks identical to yours in every respect and I labeled it an 89, presumably because that's what the owner thinks it is. He's almost certainly wrong if he disagrees with clockworkgazz of course, I'm just saying that there's a variety of opinions.
PS
shall we call you prestigegazz in future?