The format of the downloadable System 7.5.3 update differs from other downloadable updates, not least because of its sheer size in the era before CD-ROMs were ubiquitous. It is a single image of 19 parts, as the suffixes of the files show, and so designed as a convenience. In the floppy era, the update was still amenable to download and transfer to the target machine by preparation as 19 floppy disks, copied one at a time to the desktop. It was thus possible to put all 19 parts into a single folder on the desktop of the target machine's intended boot volume, and work from that, as you have surmised. Masochists could also use a single floppy disk to transfer the 19 parts.

There was also the Install CD (U96073-016A) option, on which the 16 parts are separate images. This works because the images are already together in a single folder on a single Macintosh volume (the CD). So could the separate images equally well be on 19 separate floppy disks, to be inserted one at a time on demand by the Installer application, and, very rarely, such sets turn up for sale.

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