Just add it to the "Book", by Riesch.
Parts that are seen un-commonly are not listed in the "Book". There are many examples of leave-outs. Too few known to stake you reputation on an assumption. Look up HQ stocks in the "Book", for example.
HQ hand guards are listed but not stocks. To show how rare they are, I have only had two over the years. HQ stamps have been faked out of IO stamps. I could not believe that one.
JC's problem was he had too few a sample, relied on "Collectors" he knew at the time, and when told of an error, he would not change his book. A lot of information has been added since his book was published.
At least Riesch is still up-dating his book. JC can't. But it was good talking to him many years ago. He put his phone number in his book and he answered it second ring.