I was doing some work in Exult Studio and was trying to create a new monster equipment record. I was able to create a new record which came up as the number 6088, and I was able to assign an object to it as normal. When I tried to assign the number to the shape's monster tab Equip Rec # Box, it only goes up to 100. Typing the number (6088 in my case) in and hitting 'apply' defaults it back to 100 and will not go further than 100 even if you use the arrows.
Maybe Marzo can take a look at some time.
I am not sure how much can be done here; there is only one byte for equipment offsets in monster data, so at best I could increase the limit to 255.
Fixed on next version by allowing up to 255, the maximum permitted by the underlying file format.