Siegel hopefully would only auction authentic certified examples.
We'd like to think so, but the lot descriptions for the group in this sale usually specify the date and type of certificate that accompanies. This one does not. Instead they mention that their census for the 24c SP is at about 100 examples and this one was sold by them in 1971. However, their search results don't include any mention of this item for sale 388. So, I pulled the catalog for sale 388 off the shelf and found lot 311 (realized $650), but the description mentions no certificate though several other lots mention one.
So it looks like they are expertizing this one themselves. A cautious buyer might want to put this lot on extension, even though they have a lot of experience with this stamp.