The HD motor went through a lot of design changes in the late 30's through 40's. IE, baffles, placement of forked rod, addition of oil control rings to the pistons, actually collecting oil from the engine sump and recirculating it, adding a bypass valve to the oil pump to cut down oil circulation through the engine at low speeds. Those seem to be the salient points.
They went from a design where oil left the engine out the tailpipe to actually trying to keep it inside the engine/oil tank circulation system.
Also the sales department can't be left out of the formula. It was a time where one touted your engineering advancements, and HD was perceived to be behind the crowd on that point, in that Indian and the Brits were recirculating engine oil for several years before HD started to. Now, I've never ridden any length of time behind a constant loss HD, but, I've got to assume they left a light blue haze behind them when the pump was set up right.
If you've going to tout your engineering advancements in sales literature, (recirculating oil system) I'm thinking you're going to want to ditch the "blue haze" of the earlier machines to prove the superiority of the new system. Tied to this would be an actual decrease in oil consumption as an additional sales point.
So, I'm going to assume the changes in the oiling system were to improve oil consumption, for good mechanical reasons as well as sales reasons.
Which finally leads us to the baffles and their raison de etre. To Reduce oil consumption. Likewise the rod placement, to reduce oil consumption. With the swapping of the forked rod, it help block the "flinging" of oil up onto the rear cylinder, so the baffle could be removed. The low speed bypass valve, another ploy to reduce oil consumption at low speed, helping to remove the "blue haze" from the tailpipe of a new type machine, differentiating it from older, "obsolete" machines. (sales pitch here, ditch your old VL, buy the new Improved UL).
As for one theory of the narrow slot created by the baffles creating a vacuum to help lubricate cylinders/pistions, I offer these thoughts. Ever pull the timing plug on a running harley? Get a little oil mist in your face? Right. Inside those crankcases it's a hurricane confined. Oil is being flung from the only source, the rod journals, radially, in all directions, including up at the bores. No extra inducement is needed to get it there. Witness all later Harleys with no baffles, and much higher performance and heat loads on the pistons.
They are only oiled by "flung" oil from the rod journal.
They do, however, have high quality oil control rings, which the harleys of the late 30's early 40's DID NOT HAVE.
An interesting point, that, as they existed. Hastings first marketed the 3 piece oil ring in 1935. So better technology was certainly out there.
I think that about ends the rant. In my mind, at least, it compels me to believe that the baffles were to limit oil to the pistons/cylinders.
Plus, the Old harley dealer who attended HD service schools in the 40's also told me this was what they were taught in the schools.