I sold an eyedropper pen recently with Mabie Todd’s version of the spoon feed, not unlike Waterman’s and equally efficient. Though Mabie Todd experimented with the over-and-under feed and improved it over the years, the spoon feed was noticeably better, giving good ink delivery. It was also much less likely to drop a blot.
Good though they were, those feeds were not produced for long. Mabie Todd’s R&D came up with the superior ladder feed, so good that it lasted as long as the company.