I have to agree with you.
Today packaging is so cheap and mass-produced there is no need/desire to relabel. However back in the 20s/30's this would have been common practice, it saved money, and was cheaper than ordering "virgin" packaging. Think about Henry Ford, he insisted that crating be very specific so that he could use the wood in his cars!
And as the country moved into the depression repurposing would be the norm. With labor so cheap in those days paying a guy to stencil or litho a can would have been more cost effective than ordering new cans.
Also, those who have the glass quart filler bottles know those bottles were used and reused for years, so extending the idea to cans is not a far stretch.
As to genuineness of the can, an immediate.. yep!
So, I'm pretty sure that Harley Can is the real deal.
What was more availble in the last 90 years? Water slide decals. If I was asked to place a bet, I'd bet that's an old Shell decal that's actually flaking off of the Harley Can, not under the Harley logo. That'd sure make much more sense since a decal would flake more easily that the old cans we all have floating around.
I'd also suggest that the paint and patina on that can with similar colors looks spot on with a worn Hermoline 5 gallon rocker.
I asked the seller to take the can and look at it was a magnified glass...now he does suspect that it's actually a decal over the Harley logo.
As you may or may not know, most companies didn't actually print their own cans, and many companies didn't even own the cans until they used them! When the Iowa Oil Company went bankrupt in 2004, I bought a lot of old paperwork from them. I have the original company ledgers all the way back to their first day of business in 1905 through the late 50's.
I've read through thousands of entries in that ledger, they ordered cans from the Columbia Can Company, The St. Louis Tin & Sheet Metal Works (later the St. Louis Can Company, the make of the can in question here), and many more companies.
They too did not have their own refinery, but bought their gas and oil from companies such as Cities Service, Indian Refining, Standard, Valvoline, Monarch, and many more. Likely just repackaging their items in branding cans from bulk containers. They were buying train carloads of oil and rebranding. So if you see an Iowa Oil Company can, I can be 100% confident that they didn't do more than repackage or blend the oil thats inside.
If you think about it, this is the way you buy lots of things today...you're simply paying for the brand you buy. Go to Costco. Everything Branding Kirkland Signature (or at your local grocery store that's "store brand"), is likely made by a name brand, or a company that makes "store branded" items for hundreds of companies.
Finally, I can't tell you the number of times that I've found some cans that if you looked at the paint hard enough, you could see another brand underneath it! I've seen this on original cans as early as the 20's
I've pay way too much attention to quart cans. I've seen originals in the following formats:
Metal Can with 1 Design Printed over another
Metal Can with a different brand of paper label
Metal Can with an early metal can rebranded with a later paper label
Metal can that's printed with a different oil company or beer or food on the inside.
You see the same thing with signs, Sometimes if you look at the back in the right light you'll see some sort of food can that never made it to the shelf.
I just recently bought a few cans that had nice Crown/Crystal Flash cans from the 50's that had paper labels for some additive over them. A little elbow grese and boom!
In another example that I know I've seen alot of out there are Riley Brothers Cans. Their were some green/white paper label cans that are out there that have the earlier (and much cooler) Red & Yellow painted can underneath.
In closing, I'll get off of my can box and say, that Harley can is real, and holding it in person would simply prove it.