Hi Ed,
Here is my WAG...
Just after R. Gibson's book in 1975 first came out, the collector's appetite for all lugers HK was piqued and all were snatched up.
Now those folks of "advancing years" are selling them off (through dealers) to the next generation of Owners...30 years later.
Just like my Swiss beauties...you are welcomed to buy them from me in year 2030 IF I remember you and IF I remember I even have Swiss lugers...