From memory, the brown grips were offered during the late 1980s and were not around for long, so examples are fairly scarce today. I'm not sure they attract that much of a premium though as there are still plenty of Tempests around, many in very good condition.
Personally I think Tempests should be worth more than the just over £100 that boxed examples seem to sell for but then I also remember them with rose tinted specs as the Tempest was the pistol I aspired to own ever since I saw one demonstrated in a Willesden second hand shop back in the day and back when such shops could still sell old airguns.
John
Currently looking for Baikal Makarov pistols with the following prefixes to the serial number: 98, T01, T09, T21, T22
Prefer boxed or cased but will consider loose examples too.