Aye Michael,
Anorak job here and I don't even own one of these beasties!

Using Fowlers and CMS' websites, as previously supplied, part numbers change for the rear brake disc retaining bolts in 1991 - 1992 model years.
1988 - 1991 = 90105KS6700
1992 - 1999 = 90105KAE870
Fowlers appear to give the same dimensions for both bolts, but by searching Google by entering just the Honda part number there appear to be two differing dimensions!

1988 - 1991 = 90105KS6700 appears to have dimensions of 6 x 20 link:
CMS_parts1992 - 1999 = 90105KAE870 appears to have dimensions of 6 x 17 link:
CMS_partsMy deduction would be that you have the 6mm x 20mm bolts, when you should have the 6mm x 17mm bolts!

Neither appears to be difficult to obtain.
I hope this helps.
My regards, Bill