I have received a timely, helpful answer from Library and Archives Canada.
Here it is, in part:
Thank you for your request to Library and Archives Canada regarding the Post Offices and Postmasters database.
Although the former standalone database is no longer available, the version that is now available within our Collection Search catalogue does allow one to search a partial name. To do so, please follow these steps:
1. Open the Collection Search catalogue at: https://recherche-collection-search.../Home/Search
2. Click on "Advanced Search" from the right-hand side of the page
3. From the "Database" dropdown menu, select Post Offices and Postmasters
4. In the "All these words" search box, type in the partial name you wish to search enclosed by asterisks. For example *atswo* will retrieve any results containing this series of letter (e.g. Chatsworth, Coatsworth): https://recherche-collection-search...s&q=*atswo*&