Sometimes we saw 大二爺伯 of the 5th court at CCK cemetery.
I always think that they are 谢必安 (General Chia) and 范无救 (General Fan). Perhaps each court has their own 大二爺伯 like what you say.
I used to read travels in Hell by Gigong when I was a young apprentice at a temple
Chapter 49 :
Gigong:
Hell messengers are of two type : Black and White, sometimes called 7th Master and 8th Master, Sometimes called "No help Fan", "Sure Die Chia". For those sinners, these messengers of hell will appear hideous. For those pure in heart, they will see without fear, hence the saying, if you don't do wrong, even if someone knock at midnight, one will not be afraid.
济佛曰:阴差俗称:「黑白无常」,有称:「七爷八爷」,有称:「范无救」「谢必安」,一般犯罪的人,阴差都现青面獠牙以惊吓,心虚者,胆怯脚软而受逮。若是无罪的人见之,曰:「看无妨」,相见并无干碍,所以有云:「不做亏心事,三更敲门心不惊。」如阳间犯人看见警察,好象遇见「范无救」、「谢必安」,吓得粪尿直流。警察携有手铐、枪枝,黑白无常亦携有铁链、刑具。安份守己者见之,心安理得,自然对警察视如褓姆可亲;反之通缉犯见之,以为煞星降临,先逃为妙。故善人不怕阴差,恶者见之心寒!
Those interested to read the full book of Travels in Hell (in Chinese), please refer here :
http://www.webhope.info/lcbook/Dy/Dy.html
For English :
http://www.webhope.info/lcbook/English1/E-Dy/e52a.html