Yes, the inland area around Port Elizabeth seems to be the area most Slabbert people come from during the 1800's. The Slabbert family can easily be traced back to Europe (1600's) but the 2 centuries afterwards in Africa is a bit confusing. If you are interested I can send you the Slabbert info I have collected. My family farmed in the Steytlerville area (I think). There are many Slabbert graves in PE, Jansenville etc.
I want to look further into the family crest design. Sounds interesting.