The scan resolution is indeed 600 dpi. In 2015, there was originally a high quality version of this file on Picasa (in TIFF format - uncompressed - and 2.3 MB). Much of the original resolution was lost when the TIFF file was saved as a smaller JPG (compressed) file. This might have been to e-mail the photo, publish it on a website or post it here on Rootschat. The original high resolution has now effectively been reduced to just 120-150 dpi through the JPG compression to reduce the file size.
If you still have access to the original 2.3 MB TIFF file, you'll get the best results by saving it somewhere online (like Dropbox, Google Photos) and posting a link to it here rather than posting the smaller JPG file.
Mike
Thanks. That does look a little better. Unfortunately I do not have the original available to re-scan.