It is with great interest that I have been reading Rhonda Shearer's investigative work on Duchamp's readymades. Her analysis seems thorough and her approach scientific. But the "revelation" that some of the readymades may not have actually been ready-made, that is, ordinary commercial objects simply selected by the artist, does not seem like such a revelation to me. My background in not in science, rather I am a working artist who has always found Duchamp's work to be full of humor, conceptual and visual (retinal - I know, I'm sorry Marcel) interest, and intellectual depth. Finding Duchamp's work to resonate sympathetically within me, and with a small amount of biographical knowledge (I'm getting my chronology from Tomkins biography, with the grain of salt one must take with everything concerning Duchamp), I feel like I might offer some productive speculations on process that might shed some light for the more scientifically-minded in tout-fait's audience.
Duchamp never showed much desire to repeat himself. After Nude he painted no more cubist paintings, after the large glasshe made no more mechanosexual delays. He had ideas, executed some of them to his satisfaction, and moved on. When, later in life, he did not have ideas (or so he claimed) he spent his time at chess and breathing. If he clarified his thoughts on the idea Readymade around 1915, it stands to reason (mine at least) that he would execute his idea reasonably quickly and then move on. 1915 was the year of In Advance of the Broken Arm. Even before this was the bottle rack, which he never even signed and was not in his possession in New York. (As far as the "forgery" of the shovel goes, with its square handle, which Shearer uses to call it into doubt, I cannot say much -- we are in a black swan predicament as far as proving anything goes.) But between these two early readymades (ignoring the earlier Stoppages and the bicycle wheel, which are commonly called readymades but clearly different in conception), and others we have heard of (Pulled at Four Pins) and can speculate he may have played with, Duchamp may have executed the "pure" (unmodified commercial object) readymade to his satisfaction. After all, once you have the idea, what's interesting about repeating the simple (boring) act of buying an object and signing it? He had made a readymade (in the bottle rack) perhaps even before the idea was entirely clear to him. Why do any more? Even giving a snow shovel a humorous/poetical title shows a conceptual evolution beyond the simple core concept.
It seems to me that he moved on immediately to the more interesting (to the tinkerer's mind, and Duchamp was certainly a tinkerer) project of modified readymades, such as With Hidden Noise of the following year. With Hidden Noise included the readymade aspect (buying or finding the components), assemblage, collaboration (he had Walter Arensberg put the mystery noisemaker inside), interactivity (you have to shake the thing to understand the title). Given that he had moved within months from the shovel (fall or winter 1915) to Noise (easter 1916), does it not make sense that by 1917-18 he had moved on to other ideas, which may or may not have included forgery, confusing modifications, and obfuscation of the idea that was initially (in the bottle rack) so simple and straightforward? The essential confusion, I think, is that Duchamp and, taking his lead, all his critics, lumped a bunch of disparate but related concepts under the umbrella term "readymade," (compare the early readymades to 1921's Why Not Sneeze Rrose Selavy) and while the shared classification did not bother Duchamp himself, it seems to put the taxonomists among us all in a tizzy.
As far as modification of the hatrack, bicycle wheel, trebuchet, etc. goes, all I can say is if they sat in my studio for years I'd have trouble refraining from playing with them. Duchamp had no qualms about modifying objects or documentation of objects, nor about commissioning others to physically create works for him, nor about giving misleading or false information in interviews, etc. If it gives you joy to sleuth out his "secrets," good on you. The layers of confusion are one of the gifts he gave us.