Hi flatcat,
When the 3525 fire ladder trucks & 3526 fire engines were both in production, they came with one decal sheet that had 4 white unit number 21's & 4 white unit number 23's included on the same decal sheet. To be politically correct to match the original box, the 3525 ladder trucks were numbered with 21's & the 3526 fire engines were numbered with 23's, 2 on top of the rear cab & 2 on the front sides. Someone must have wanted to have a 23 unit number ladder truck before you got it!