Long box LB, Short Box SB, Long Narrow Box LNB, Short Narrow Box SNB. I think that about covers it.

I would think long wheel base or short wheel base if SB. Meaning bed of the truck and frame different length. But that's when shopping for one. Not sure if it was something you seen in an add or if it's abbreviation for something mechanical

Then that would be something like: SBC for small block Chevy or BBC for big block Chevy then you would also see SB for short block or LB for long block. Fords would be SBF and so on.

But they normally list then as long box or short boxes in the advertising.