The “standard” pronunciation of lb or lbs is the same as for pound(s).
However, given the nature of humans, I find it likely that in some slang a pronunciation based on the written word is used, e.g., /lɒb/. I am curious whether my guess is correct, i.e., whether such a pronunciation exists.
Unfortunately, searching the Internet for this did not give any satisfying (positive or negative) results, as it primarily yields non-slang dictionary entries, which only give the standard pronunciation.