Have you ever noticed the little white line on the front edge of the Dual TK-12 cartridge holder? Most users haven't noticed and if they did they don't care. Still a few others think this is a center line. But if you look closely and in fact measure the line's position you will find it is not in the center of the cartridge holder. No sir, the line is actually 2mm to the left of center!
Was this a mistake on the part of Dual when they designed the TK-12. Hardly. The line was purposefully set to the left of center to aid the user when positioning the tonearm on the record. By off-setting the line the user was less likely to miss the edge of the record when manually cueing the tonearm and hopefully preventing a mishap which could damage the stylus.
The cartridge holder doesn't need this line but if you have an original Dual cartridge holder that does know that it was positioned off center intentionally.