H&R was the USA HK4 importer up till the mid 1970's.
Single caliber, multi-caliber sets and even a 100th yr commemorative (H&R's 100th year!)

I just looked at a HK4 this weekend. The frame was cracked where the small steel insert was imbedded to limit the rearward travel.
I get the feeling it's not a rare condition on these .