There were two different plates depending on the model year. The earliest Monsters had 43310281A
And the later models, (including your 1998 Monster) used 43310281C.
As you can see, the level lines are different on the two plates and I think the higher lines represent about 250ml additional oil.
Your plate is apparently still available and, according to Ducati Coventry, is only £3.90.
The plate mounts between the two raised lines on the crankcase.