The reason for the metal rings on some calipers - there are two types of 13WG

I compared, and some of the 13WG calipers have a ridge to retain the dust boot, and some do not. The ones that do not have a shorter shoulder, so they really need the ring. The ones with the retaining ridge do not. They also use a different style boot.
That's the reason for it, not a manufacturer screw up.

 

Here's more on the caliper seals:

The 04479-60080 kit is for calipers made between 9-2004 and 1-2006. These do not use the metal ring; instead, there is a retaining lip on the cylinder body.

Calipers made after 1-2006 use part number

$25 04478-04050 Toyota Genuine Parts - Cylinder Kit,Fr Disc (04478-04050)

04478-04050. These have no retaining lip, but instead use metal rings to secure the boot to the (shorter) cylinder shoulder.

There's also a combination rebuild kit, number

$29 04479-60081 TOYOTA Caliper Repair Kit - 04479-60081

04479-60081 that includes both types.

So, if you buy a reman 13WG caliper, it may be either type, seals with rings or without.

Hope this helps someone out. It can be confusing to have two versions of a part that is interchangeable.