6497s and 6498s don't officially hack (the fourth wheel, and by association the subseconds hand doesn't stop turning when the crown is pulled to the setting position, which makes syncing the seconds hand almost impossible). It has been reported that one can fake a hacking behavior by putting back pressure on the crown in the setting position. Without actually rotating the crown, and therefore without actually changing the hour/minute setting, you can use this trick to make the fourth wheel stop turning. When this works, you can actually see the balance rapidly spin down and stop, everything completely stops. The moment you let go of the crown, everything starts again (in my experience you can deliver a slightly more solid initial "kick" to the seconds hand by rotating the crown forward just enough to fill the gear-slop). In this way, you can sync the seconds hand on a nonhacking movement, or least you can on the 6497 and 6498.
I have several 6497s and 6498s and some clearly support this trick while others clearly don't. The ones that work, really really work. The halting effect is almost immediate, the effect is total (all motion stops) and the effect is just as easily and unambiguously released. Likewise, the ones that don't work, really really don't work at all, not even the tiniest little bit. With a little back pressure nothing happens at all. The obviously tempting stronger back pressure consequently begins to turn the hour and minute hands the other direction, and even at that point the fourth wheel and balance still continue forward unabated at full speed.
I haven't taken these movements apart yet to see if there is a difference, but I thought they should all be mechanically identical. Does anyone understand what's going on here?
The movements that support this behavior are an unbranded "swiss" movement (from one of the common gold Arnex pocket watches from the early 80s) and a SeaGull 6497 and a SeaGull 6498. The movements that completely fail this behavior are a Hangzhou 6497 and a Hangzhou 6498.
Any idea what accounts for the difference here?