The original cab 20 is not the most rigid frame, so do be careful when stringing. I have strung one of mine to 27lb before but as I no longer play with them, my other three were just kept as part of the collection and restrung to 21-22lb.
Check around the frame to see if there are any significant "section loss" chips/scratches... and also any cracks. Many cracks tend to propogate from grommet holes so check all the grommet holes for any cracks.
Finally, if you decide to restring, make sure it is done on a 6-point support machine and by an experienced stringer. Replace any damaged grommets and if you get along with them, try and use a thicker string to reduce the stress on the grommets.