7.5 was *NOT* available on 800K floppies. The Plus is the only system that can run 7.5 that was "guaranteed" to not have 1.4 MB drives, as the SE and II were available with both 800K and later with 1.4 MB drives; and both had 1.4 MB drives available as an upgrade after-the-fact.

The most common methods for installing 7.5 on a Plus were CD-ROM and network.

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Well that makes sense. 7.5 came out quite a bit later than the Plus was canned. But certainly 7.1 was released on 800K disk if this Apple document is to be taken at its word: http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English-North_American/Macintosh/System/Older_System/For_System_7.1.x/System_Update_3.0_800K_Disk.txt

But I wouldn't place too much stock in Mac IIs and SEs being upgraded to Superdrives. I never saw any contemporarily during their availability. The Mac II (Superdrive), like the SE FDHD was probably not offered until well past the release of the SE/30 and IIcx models to encourage machine replacements rather than less lucrative upgrades. Both the SE & Mac II only sold for a year in Superdrive configuration in any event, so the vast majority would have been 800K drives. But I could be wrong. The more likely reason as equill hints is 7.5 didn't really run on 68000s and 68020s well – after all the last 68020 LC was discontinued well before 7.5 was introduced, along with the last 68000, the Classic.