The first question I had to ask myself was does the juvenile really have a short bill? In fact the bird is a particularly small, slight individual compared to the two adults and this alone would account for the shorter bill in direct comparison. However the bill is about 1.3 times the head length, despite the head itself looking somewhat small, so I'm sure it must be on the short side for Long-billed. However I don't know how short a juvenile's bill can be, particularly if a male, but an online search quickly reveals equally short-billed birds. So short but not exceptional might be the answer to my question. I gather only the very shortest Short-billed Dowitcher bills are identifiable by bill length and I'm curious to know what they'd look like but I've yet to find an online image purporting to show a Short-billed identifiable on bill length alone. Though the deeper base looks obvious on some individuals.
While I have no doubt about this bird being Long-billed (even if I'd hoped otherwise when I first saw it), the only very slightly marked tertials and inner greater coverts immediately place it firmly in that camp. Nevertheless it remains an odd-looking bird to me. The proportions don't quite accord with the usual fairly consistent Dowitchers I see. But of course though Dowitchers can look straight-billed one minute then droop-tipped the next, this bird takes variability to a new level and it could move higher or lower on the scale of oddness as if outrageously attempting to morph from Dowitcher to tringa at times.
Some of the following images were taken early in the morning before the sun broke through, others later in the day with bright side lighting. The selection I'm posting here attempt to show the bird from all angles and give a sense of what to me, with my very limited Dowitcher experience, is a peculiar bird. Perhaps people used to seeing them will wonder why I find it unusual.
Long-billed Dowitchers; winter adult left, juvenile right. It looks more like a tringa to me in this shot, with the shortish bill, head shape and eye position with the bold cap and striking, straight eye stripe and supercillium it could almost pass from a Wood Sandpiper!
In profile the bill is short compared to the head but notice the tibia too. Whether relaxed or alert the visible leg between belly and knee often looked short whereas the other two didn't vary much. The fore-crown feathers were frequently raised imparting a quite different head shape.
When alert its slim build became even more obvious.
Though it never showed more than two primary tips beyond the tertials it did so often, a small projection can be seen even on a 'distant' view like this. Those of the adults were more often cloaked. The following shot shows how prominent the projection could be.
The primary projection, though short, was often surprisingly obvious, perhaps due to growing tertials?
The shorter-billed of the two adults with the primary tips only just visible.
The warm wash across the breast and down the flanks with prominent spots (almost streaks) make the underparts as strikingly different as the upperparts compared to the adults. 
The tertials and inner greater coverts have only a single rufous internal mark each (the former with even fringes), comfortably within the bounds of Long-billed appearance.Trawling through Long-billed / Short-billed identification was something of an education but I was left puzzled by a couple of points this threw up. One was the loral angle and supercillium shape and the other the white lesser underwing coverts of Long-billed.
I found the former puzzling because the angle and shape varied so much depending on stance. The first two or three shots in this post show an almost arrow straight line, straighter even than I'd expect from the average Long-billed, while the following shots show a distinctly arched Supercillium. Then there are the underwing coverts. Both the juvenile and one adult have fully barred lesser coverts so presumably this is a one way feature; if there's a white bar it's Long-billed, if not it could be either?
Another feature my inexperienced eye was drawn to was the significant difference in rump barring. The juvenile has very fine barring but the adult has very bold, broad bars. Is the fine barring a feature of juvenile? I can't find any mention of this in the literature I have. Nor did I find any mention of the rufous tips to the tail and longest uppertail coverts outside of breeding adults. This strikes me as surprising because so much detailed information has been written about this difficult to separate pair. Perhaps because these features have no relevance to the separation of Long- and Short-billed? It never ceases to amaze (and frustrate) me that after a lifetime birding there are so many things I don't know.
Compared to earlier images the supercillium in this shot is very arched, broad before the eye... and long. 
Here the supercillium is between the extremes.
The longest uppertail coverts and tips of the central tail feathers are quite rufous.
The juvenile. 
The juvenile's inner secondaries are barred white throughout their length, the adult has a single bar but I don't know if this is a feature of juvenile or just individual variation. Likewise with the much finer rump and uppertail coverts barring, very different to the adult below. 
Adult; central tail feathers dark and relatively uniform, rump and uppertail coverts boldly barred. 
Juvenile with barred lesser coverts.
Adult with barred lesser coverts and dark tail.
If it looks like a rock, stands like a rock and sounds like a rock... it's probably a Pintail.
