Why delusive? Because it traps you into thinking that triplets start in a different place. Have a listen at two examples, one by Chet and one by Tommy.
In this video the lick is at 2:31. It contains three triplet groups.
In this video a very similar lick starts at 1:39. It contains four triplet groups.
When I first stumbled upon it seemed obvious that the roll contains triplets played on three strings by repeating thumb, 1st and 2nd finger. Something like this (hold Emaj for this ):
However no matter how long I tried to time above licks in this manner I always finished earlier than the original. It took me some time to find my mistake. And the mistake was the first group. It actually isn’t a group played on three strings. There is a hammer-on there so the first two notes are played on the same string. Here is a rough transcription of those two lick:
Chet’s lick starts on the second quarter note (on two). You play open 6th string with your thumb. Then goes the hammer-on and then the 5th string which is the last sound of the first triplet. It means that you begin second triplet with the fourth string plucked by second finger. The lick is based on C/G chord (C with G as the lowest note). Both 6th and 5th string are covered by third finger but you can use 3rd and 4th.
Tommy’s version is a little longer. It starts on one but the rule is the same. Chord is a bit more complicated. It’s C11 with F in the base and #5. you grab 6th and 5th string with your thumb in this one. Put your capo on 2nd fret if you want to play with the recording.
What fooled me in the beginning was natural grouping imposed by right hand fingering (thumb, 1st, 2nd). If it’d coincided with the triplet groups it would have been easy but at the same time it wouldn’t have finished on the first note of new bar (on the beat). By beginning with two notes per string and only then switching to normal right hand pattern the last note of the right hand group is also the first note of another triplet group or of anything that comes after the lick. Be finishing on the beat it it has more final sound to it.
This pattern is easily applicable to any chord as long as you can put those two notes per string in the beginning. The lick sounds good at the end of a phrase.
As a dessert, for comparison let’s look at very similar lick played by Buster B. Jones (1:56):
This version has normal beginning and so to end on the beat the last note of last triplet is repeated by the thumb.