It is no less easy to find that other stamps of that era. The problem is having folk list it for sale or for dealers at shows (those pre-pandemic gathers of stamp collectors and dealers) to actually carry the item to the show. Like most Christmas stamps used in mass mailing Christmas cards, they tended to be avoided by collectors due to the high usage. That said, unlike other stamps of the period, the gum tends to discolor the stamp which cause some to be tossed. Now the main use was intended for the test cities, it was available nationally.
Finally as a pre-cancelled stamp, use was later restricted. But at first those without pre-cancel permits could use the stamp as if non-precanceled.
In a pure business model you have high supply, low demand ratio and thus not an item to spend much time working up.
All that said, I have tracked this issue for a while and believe usages as part of registered, certified, international, priority and the like are quite less common than compared to other issues of the period. So you can find first class usages, but putting together a collection of other usages can be a challenge.