It has been said that "curiousity killed the cat". Even so, I'll ask even though I suspect that all the old-timers already know the answer.
In numerous places in this forum Walter B. has proposed that HP market a new calculator which he has dubbed the HP-43. While thinking about that I realized that there has been a real dearth of HP machines where the second digit is a three; e.g., as I go through Wlodek's book there is no mention of an HP-13, HP-23, HP-43, HP-63, etc. Maybe the answer is in Wlodek's book and I just haven't found it. So I'll ask "Why is that?"