The term "bookends" AKA "shoulder notes," is usually known as Gary_T described: two notes the 1 before and after the radar, for consecutive note collectors. Bookends always come in pairs (just like the real bookends)-- not singularly.
I don't have a picture about a bookends note.
For example, there is follow numbers.
8500085 ---> 85 000 85
1200012 ---> 12 000 12
0600006 ---> 06 000 06
7000070 ---> 70 000 70
....
They are not a radar or shoulder note.
Think meaning of BOOKEND
I've never heard of bookend serial numbers, which seems to be the concept you are introducing here (a note which stands on its own merits). To me, this is a non-sequiter, since the logic of applying a bookend concept to a single note does not follow.
Occassionally, I've heard the "8500085" referred to as a "repeater" since the number 85 is repeated and there is symmetry in the note. Hence 7000070 would be a 2 digit repeater, whereas a 0700070 is a two digit radar. Typically the premium for a 2 digit repeater is less than the premium for a 2 digit radar because there's less symmetry in a repeater (yet both are valued for their unique numbers).
