Michael, the two postcards you posted are amusing to me, because I grew up in Minneapolis, one block from Lake Harriet-- which is one of the three central Minneapolis lakes. The postcard-makers tacked the names of Minneapolis lakes on places that don't look like Minneapolis to me (I've seen others too), and my guess would be the one with the Minneapolis label is "the wrong one". Maybe they both are...
Rhinelanders are rare outside Wisconsin, or so I've been told. Dunno if any made it to Belle Isle, but one thing to consider is that most of the canoes paddled in Belle Isle's canals were livery canoes. While it is possible that individuals traveled to Boston and Rhinelander, bringing home canoes that they paid to store on the island, I doubt liveries were filled with canoes by these builders. It would have been simpler to order them from the Morris dealer, Charles Molitor... at least, that's the story.
Charles Molitor bought used canoes, and perhaps some of those made it into his livery... but I have assumed the used canoes he purchased were re-sold, because my impression of his livery is that he wanted canoes with 36 inch bow decks and 24 inch stern decks, and a lot of mahogany.
The profile of the Morris canoe changed over the years. Once he came out with "special ends" (torpedo stems), it seems Molitor ordered that type for his livery. Our 16XXX Belle Isle Morris has special ends, and appears, in profile, like canoes that graced the Charles River. Earlier Morris canoes in Molitor's livery would have had a different profile.
Belle Isle is a park with man-made canals... a small place when compared to the Charles River. It held a finite number of canoes, even when all jammed-up with folks listening to a band concert. I'm not trying to argue that there couldn't have been canoes built by a number of different builders, just that it wasn't like the Charles in that respect.