David.
Your bold font does mark the correct spacing of the notes played for quarter note triplets. If the first half of a bar is taken up with triplet eighths then you have: 1 - trip - let - 2 - trip - let
Three notes occupy the space of one quarter beat. Or, six notes occupy the space of two quarter beats. Let’s work with the idea of six notes.
Six is divisble by three meaning if we are going to split these two quarter beats into triplets, each will take up two-thirds of an eighth note triplet.
sixth sixth ... sixth sixth ... sixth sixth
two thirds ... two thirds ... two thirds
The triplet eighths are counted as 1 - trip - let - 2 - trip - let. Placing them within a similar framework allows us to see the connection between triplet eighths and triplet quarters.
I have created an audio and TAB in Guitar Pro showing step by step how you can arrive at quarter note triplets from eighth note triplets. I have displaced your pentatonic run to some notes higher up the neck, in minor pentatonic pattern 4. We will eventually come to a bar with the exact same rhythm as your ‘unusual bar’.
Bars 1 and 2 contain no guitar, just a drum beat playing quarters to establish the pulse.
In Bar 3 are two eighth note triplets followed by two quarter notes (the latter of which play the same pitch - the root note of A minor pentatonic).
The rhythm count is → 1 trip let 2 trip let 3 4
In Bar 4, the triplets have been modified. The middle note of each triplet is played as a muted string. There is still a separate sound to allow us to hear the three parts of the triplet, but it has no pitch.
The rhythm count is → 1 mute let 2 mute let 3 4
In Bar 5, the triplets have been modified further. The middle note of each triplet is now a rest - leaving a space between the first and last note of the triplets.
The rhythm count is → 1 ___ let ___ trip ___ 3 4
In Bar 6, the two spaces created within the two triplet eighths of Bar 5 are now filled. This is done by extending the duration of the preceding notes from single triplet-eighths to double triplet-eighths. Double triplet-eighths are exactly equivalent in duration to triplet quarters.
The rhythm count is still → 1 ___ let ___ trip ___ 3 4
In Bar 7 the two quarter notes in the second half of the bar have been tied to make them sound as one half note.
The rhythm count is → 1 ___ let ___ trip ___ 3
In Bar 8, the notation is changed to reflect that final note lasting a half bar, and it is now shown as a half note, not two tied quarters.
The rhythm count is still → 1 ___ let ___ trip ___ 3
In Bar 9 the final modification is made. We have three notes occupying the space of two quarters. Or, if you prefer, three quarter note triplets occupying the same space os two eighth note triplets. It is all the same. These quarter note triplets were arrived at from the more familiar eighth note triplets which is why the naming of their count is a little off kilter part way through the process. We have the rhythm count as 1 - let - trip. However, because these are now firmly established as quarter note triplets, we can reset the rhythm count. We dispense with the legacy of them coming from eighth triplets and now describe their rhythm in their own right.
The rhythm count is → 1 trip let 3
Here is the full tab.
Here is the audio with a tempo set at 60 bpm. Listen as you read the TAB.