Depending on which model of "hook wrench" (C-spanner) came with your bike, it may be too short to be used as dipstick to check the oil level in the gearbox. I have a 1992 K100RS and the handle of my hook wrench is only 10 cm (3.9 in.) long, too short to reach the proper oil level contrary to the instructions found in the rider's manual. Also, my hook wrench doesn't have a marking line indicating the maximum oil level, which is a dead giveaway that the hook wrench should NOT be used to check the oil level.

I suspect BMW replaced the hook wrench sometime around 1992, from a model with a longer handle and a marking line (part #71112315446) to a model with a shorter handle and no marking line (part #71112315897). Unfortunately, BMW didn't update the rider's manual to reflect the change and they didn't provide the L-shaped dipstick that replaced the longer hook wrench to check the oil level on K100RS 16V models. It's possible that the original owner of the bike lost the dipstick but I don't think so. The instructions in the rider's manual are wrong anyway.

In any case, if you are like me and you have a newer, shorter hook wrench and no dipstick, you have two options: you can order the dipstick (part #71112316195) from BMW for about $10 or you can make your own dipstick using the specs provided by Rick in this post:
https://www.k100-forum.com/t13285-c-spanner-needed-has-grown-legs-or-is-champion-at-hide-and-seek#157051

As Rick stated in his post, you must make absolutely sure that your makeshift dipstick cannot fall into the gearbox, be it L-shaped like an hex key (Allen wrench) or fitted with a handle larger than the filler hole like a large screwdriver. Note that the MINIMUM level (tip of the long leg of the L) is 12.3 cm (4.8 in.) from the filler hole with the dipstick as opposed to 12.6 cm (5.0 in.) with the old hook wrench. The MAXIMUM level is unchanged (11.6 cm or 4.6 in. from the filler hole) corresponding to 0.8 liter (27.1 fl. oz.) on a 1991/1992 K100RS.