You might targeted your letters by mentioning the company by name, but you were promoting the same skills and experience, so they weren't actually all that different. (sciencemag.org)
The more you know about them, what is going on in the company at present, what their needs and goals are, the more target a letter you can write. (thecareeradvicecentre.com)
Get right to the point and write short targeted letters. (thebalance.com)