I bet it might be in different places on the different Bolers (since they were manufactured in a bunch of plants).
On my 1974 Neonex (Winnipeg) we couldn't find it on the frame anywhere (as I'd' read about), but I did find it in three other places. Something to check, anyway:
1) On a "metal" (actually I don't think it's metal but it looks like thick aluminum foil) placard that's just below the belly band about halfway along the driver's side of the trailer.
2) On an English paper placard inside one of the overhead kitchen cupboard doors (also lists appliance data, etc.).
3) On a French paper placard identical to the English one inside another one of the kichen cupboard doors.
Mine is a number like B 130 B xx xxxx. Only the last six digits are really the serial number (the first four are model number stuff), but the PO's title
listed all of the numbers under serial number since they are all written together on the "metal" placard. I kept it like that for the new title
New trailer? If so, congrats!