Alternatives if you don't wish to use ua608d to uncompress “SSW_6.0.8-1.4MB_Disk1of2.sea.bin”, when following the Getting started with Mini vMac instructions:
Those instructions used to suggest using any modern version of the program Stuffit Expander (now from Smith Micro Inc). But the current version no longer supports uncompressing this archive. If you can find an old enough version of Stuffit Expander, that can work.
Or, you can use an old Macintosh to expand it (i.e. such as your Macintosh Plus.) The file “SSW_6.0.8-1.4MB_Disk1of2.sea.bin” contains a Macintosh file encoded in MacBinary format, preserving Macintosh specific information. You can use a program such as "binUnpk" to extract the program “SSW 6.0.8-1.4MB Disk1of2.sea”. This program is a Stuffit Self Extracting Archive. Running it on an old Macintosh will get a disk image file named “System Startup”.
Instead of a real Macintosh, you can use the program Executor, which can run some 680x0 Macintosh software without requiring any software from Apple. Though it is no longer developed or supported, it currently may be downloaded for free. It comes with StuffIt Expander 4.0.1 which can extract the program “SSW 6.0.8-1.4MB Disk1of2.sea” from “SSW_6.0.8-1.4MB_Disk1of2.sea.bin”, and then “SSW 6.0.8-1.4MB Disk1of2.sea” can be run to get the disk image “System Startup”.
If you happen to have any sort of disk image that can boot Mini vMac (i.e. containing system 7.5.5 or earlier), then you can use Mini vMac to extract the archive, to get a clean copy of official Apple system software. You will need a large enough blank disk image (see “Blanks”), ImportFl to import the '.bin' file, binUnpk to decode the '.bin' file (to get the '.sea' file that you run), and ExportFl to get the extracted disk image back to the host computer.
“Samir” points out that The Unarchiver, a free and open source utility for OS X, “can extract .sea files as well as .bin files and a lot of other archive formats”. ua608d descends from the command line version of The Unarchiver.
“Bjoern” points out another alternative for Intel Macs. System 7.0.1 is available from Apple in '.smi.bin' format. '.bin' can be decoded by any Mac version of StuffIt Expander, and probably other programs as well. This results in an '.smi' file that can be opened in OS X, mounting a disk image that contains 6 disk images that can be used in Mini vMac.
Previous versions of the getting started page recommended using “SSW_6.0.8-800k_Disk1of4.sea.bin” through “SSW_6.0.8-800k_Disk4of4.sea.bin”, which have the same system 6.0.8 software, and which are also available from the Apple. Thanks to Ryan Schmidt for pointing out that these 800k disk images are packaged as DiskDoubler Self Extracting Archives, which can not be expanded with Stuffit Expander, while the 1.4MB disk images are packaged as Stuffit Self Extracting Archives, and can be. The 1.4MB images can not be used with a real Mac Plus, but the emulated Mac Plus of Mini vMac can use them, because of the replacement disk driver.