OK. The nuances that make this a rare Vacumatic
We will build up to it with some background.
The only catalogue-illustrated triple-band standard size (5-5.125" by 0.5") Vacs are the first generation Vacumatic Standard, and the briefly-made predecessor Vacuum-Filler Senior.
1st Gen pen: Vacumatic Standard: 1933-1938

With onset of the 2nd Generation of Vacs, the high-line standard size niche was filled by the Major, not a triple band pen.
2nd Gen pen: Vacumatic Major (relatively early form of this tricky model) 1937

By late 1939, through early 1942 (the start of 3rd Generation), Major began to appear in the slightly longer rod-stock previously reserved for Slender Maxima. I labelled these "Long" Majors to differentiate them from prior and later pens. This is same model same price sticker as the pen just seen, in slightly later trim and- if you look closely- in slightly longer stock both cap and barrel.
2nd Gen Pen: Vacumatic Major 1938-1941: "long" variant.

The long rod stock for Major persisted into start of 3rd generation, so early 3rd Gen single jewel Majors (early-mid 1942) are Long Majors too. Here is a correct early 3rd gen Long Major
3rd Gen "Long" major 1942

By late 1942-1943 Major returned to its 1937-8-ish Short rod stock and would so continue through end of USA production in 1948
3rd Gen "usual" Major 1943-1948

Both 2nd Generation and 3rd Generation Vacumatic Junior (a non-lifetime standard size pen) were done in the typical "short" rodstock, even when Major was going "long". Junior is best known by its double cap-band. As follows:
2nd Gen striped Junior 1939-1941

3rd Gen striped Junior 1942-1948

Confused yet?
OK. Let's return to triple cap-bands. As noted, the only catalogued standard-size triple cap-band pens were the 1st Gen Standard (and its nearly identical if differently named predecessor, Vacuum-Filler Senior)
1st Gen Standard

But, around late 1938-1941 Parker- using the same "long" rod stock that was original to Slender Maxima and had been later used for "Long" Major- produced a 2nd Gen Pen with triple cap-band that essentially was a streamlined Standard, much as the entire 2nd Generation of Vacs were more streamlined than first Gen pens. We've labelled this uncatalogued pen- not surprisingly- Streamline(d) Standard. It was a gray-zone model, featuring clip evolution like an economy-line Vac, but having initially the two tone nib, striped section and striped jewels of a high line Vac. Charming model available in five colors and three clip styles for at least 15 pens.
2nd Gen off-catalogue Streamline(d) Standard

Again, compare it to the Long Major. Same rodstock
Far less common than the 1938-1941 "long" rodstock Streamline(d) Standard are some occaionally seen 3rd generation "short" rodstock pens with triple cap-band. We are coming home now.
Take either the shorter 3rd gen Major (blue diamond) shown below, or the Junior (non-blue diamond) shown below and imagine with triple cap-bands and non-BD clip, producing a 3rd gen "short" Streamlined Standard
Major:

Junior:

With triple Bands... Streamlined Standard 3rd Gen: I don't have the "catalogue" style shot for one of these just yet, but here is the original pic (same size pen as the above two) in green. This is not the well known (if uncatalogued) 2nd Gen cap frankenpenned onto a later barrel. The cap has both short rod-stock dimension and the olive-green color of the later green Vacs (vs more bluish-green of pre-1943 pens). The short 3rd Gen pens with this band are far less common than the already uncommon 2nd Gen pen shown above, in red.
OK. So... issues of scarcity???Here's the deal. These late "short" Streamline(d) Standard vacs appear usually with 1945-7 date codes, iirc. This does not disprove continuous production from 1941-2, but might represent small sampling available. Of course it is possible Parker returned to this cap-band after a gap. We don't have info regarding this.
And, Whilst I have handled probably 20 pens of this sort over the years, they almost all are Black or SIlver Pearl. I might have a brown lying about (need to check). I have seen one cap previously in green and now this pen. So... having photographed 6000 vintage pens over the years and thus having had chance to handle truly the pens of great rarity and cachet, owning- idunno- many hundreds of Vacs now not to mention the hundreds sold, this is maybe the 2nd green, uncatalogued short Streamline(d) Standard I've
ever seen.
No doubt others are out there buried in collections, underappreciated for their scarcity. Such is the charm of Vacumatic collecting.
The scarcity of green in this trim also points to a pet issue in my Vac collecting... the underrepresentation of Green amongst off-catalogue cap-band styles. That is a chat for another day.
As per my original post, the question of "who cares" is fair, but those of us- the few, the proud, the crazy- who live to map out the oddities of Vacdom- like this stuff.
regards
david