[sdiy] Extra designations in transistor part number

[David Ingebretsen]

I need to buy some BC550's and BC560's. The schematic calls for a "C" class,
so I need 550C's and 560C's. The ones I can get at Mouser have extra letters
BU after the C. I assume these refer to temperature, or some other spec. but
I have come up short trying to figure it out.

I've looked at a dozen different datasheets, but they only clarify the
difference between the "A", "B", and "C" classes, not the extra "BU"
designation.

What is the difference between a BC550CBU and a BC550C?

Thanks
David