Last updated Jan 30 2024 (c) Herb Johnson

The Transit Protector, is a plastic device the size of a 3.5" diskette.
Supplied to Apple from Sony, it's yellow plastic, with a thin section
of plastic where the heads rest when inserted in the drive.
 
Apple documents I've read, (128K, 512K, 400K and 800K drives) all refer 
to this device as a "packing disk". They give a part number but it's not 
upon the device and clearly it's a non-typical Apple part number.  

Generally they came with external 400K 800K floppy drives or new early 
Macs. It's hard to determine exactly which Apple products were shipped 
with it, Apple documents did not always call it out when it was included. 
I've seen it with 400K Apple drives and 800K external Apple drives. 

One document with much to say about them is the Apple document for the 
800K external disk drive:

https://mirrors.apple2.org.za/ftp.apple.asimov.net/documentation/hardware/storage/disks/Apple%203.5%20Disk%20Drive%20Owner%27s%20Guide.pdf

Apple 3.5 Disk Drive Owners Guide:

4. [After connecting the drive to your computer,] Turn on the computer's power switch.

A yellow plastic packing disk will eject. This disk was inserted at
the factory to protect the drive mechanism during shipment.
See "Removing a Disk" if the yellow packing disk doesn't eject
automatically. You must remove it before you can operate your
drive-

5. Gently pull the packing disk all the way out of the drive and keep it
for later use. Use this disk to protect your drive mechanism whenever you
transport your drive.

Removing a disk

Before you remove a 3.5 inch disk, make sure the drive's "in use"
light is off so you wont lose any data.

To remove a disk when yar aomputer is on, push the eject button.
Then gently pull the disk out of the slot. The eject button works only
when the power is on.

When the power ls off: If there is a power failure, or if the e.iect button
doesn't release disk, yqt can rernove the disk from the drive
manually. To remore a disk manually, use a straightened-out paper
dip or similar object. Insert it into the acess hole on the eject
buttorL Push hard on the paper clip until the mechanical eject 
mechanism relased the disk. (if the disk doesnt come out, push
harder so you release the spring insid the drive-)

Your disks are designed to be easily r emovable. You should not
have to use force. If a disk gets stuck, see your dealer. You can lose
disk and damage both the disk and the drive mechanism by forcing
a disk out. 

Your drive is making errors.

Make sure you have not blocked the air vents or placed the drive on
top of the Macintosh where the electromagnetic field will interfere
with the drive operation.

Caring for your disk drive

When you transport the disk drive, insert the yellow packing disk
to protect the drive's internal mechanism. (The magnetic heads
in the drive have hard ceramic surfaces that could crack if they
contact each other.) If you lose the yellow disk, use a blank disk
during transport.

end of quotes

https://downloads.reactivemicro.com/Apple%20II%20Items/Documentation/Manuals/Apple%20Service/Apple%20Technical%20Procedures%20-%20Cross%20Family%20Peripherals%20-%20Vol.%201.pdf

Apple Service Technical Procedures (march 1991)
Cross Family Products
Volume One 
PN: 072-0230

Drive Mechanism

Packaging

1.4 1 Basics

When sending in the Apple 3.5 Drive 800K mechanism
for exchange, it must be shipped in the Apple-approved
packaging, which includes the shipping fixture and the
protective packing diskette. Refer to the Illustrated
Parts List for additional packaging information.

FINISHED-GOODS ASSEMBLY (Figure 1) [parts list and illustration]

003-0003 Packing Disk (for transporting)