Well, you know how to entice me to join this little forum of yours. But really David - you revealed most of this in the FPN post, so the little teasing shots seem a bit redundant. We've seen it before and then some.
As for what is early or late for DM, it is all cataloged. If there is one thing we know about these Sears pens is that they were well cataloged, and the catalogs are not too hard to find (though I confess I have not had the library time that I wish I had).
Not to spoil the fun, but here are some of the catalog pages from Spring 1935, Fall 1935, and Spring 1936. I have some 37 or 38 cat. pages in my files at home, but don't have scans with me at the moment. I don't believe that the "web" or brick pattern appears in the 38 catalog, so that would give you an idea of "late".
Some additional observations on the various rebadged Parkers:
In addition to the date codes, which also appear on challanger-like mid-30s Webster and possible Good Service pens (have to check the latter), the Diamond Medal pens also advertised the 2-sided nature of their nibs.
Websters and DM pens generally maintain a 3-band style throughout the line.
The first year or so of DM pens saw "Sacless Vac-Fill" pens being sold alongside button-filled Diamond Medal "Non-transparent Sac-fil" pens with the same colors and trim, and only $1.00 less. So while the Parker brands kept the Vac-fil as a unique higher-tier pen and the button-filled Challengers as a tier below the Vac-fil pens, the DM pens mixed the two.
Sears higher-quality pen brands at this time period consisted of Diamond Medal, as a top-line, Webster, as middle-line, and Good Service, as a lower end, but still quality pen (mostly, though not always 14K nibs, and price-point around $1).
Sears also used the budget brands Truepoint and Tower-Truepoint, later just Tower (playing on the Sears Tower, which at one point was the tallest building in Illinois), as well as un-branded budget pens. Some of the allegedly unbranded pens had Ambassador levers. Some of the Truepoints are clearly in the same family as the other presumptive National Pen Products co. Pens, others are identical it Eagle pens of the time. I don't think any of the Truepoints were made by Parker (but if the Fifth Avenue pens, by the "Stafford Pen Co." were made by Parker, I suppose anything is possible).
Finally, on the Fifth Avenue Pens - these were in fact made by Parker for Woolworths (at least in the early-mid 1930s). Not only were they different than other Parker brands, they were made with reclaimed materials and scraps, and employees that made them were paid less than the mainline Parker employees. Austensibly the reason was just to try to provide some sort of jobs to people in the bottom of the Depression. It also seems to have been a starting position, and better performing employees could be moved to the main Parker products.
I think the story behind the Fifth Avenue Pens (which is based on a shop-floor employees recollections, so should not be assumed to represent Parker Management thinking) gives a clue as to why Parker was willing to sell rebadged Parkers through outlets like Sears and Woolworths. It was the depths of the depression and companies like Parker were desperate to get whatever market share they could. The Sears house-brands were probably a ripe prize, and at $4.95, they were not that much less than the Parker Juniors.
Spring 1935
The second pen from the left (already sold) in Richard Binder's Pen Tray for August is a silver pearl Parker/Diamond Medal. Is it Shadow Wave plastic or something else entirely? Would love to see a close up...
It is not a Shadow Wave plastic, but something else entirely. It is a striated grey, similar to the Sheaffer grey striated pattern, but more stripe-like - see the funny green striped Parker at the beggining of this thread for the green version. I think I have it in the 1938 catalog, but have to check when I get home. I have one of these in grey the slender size, incidentally, though I haven't had a chance to shoot a pic.
Also, here is a picture of the faceted button filler that appears in the Spring 1935 catalog page. I don't think Parker made anything comperable (fluted, but positioned as a top-line pen).