I sometimes wonder why people made the guides...
https://www.omegaowners.com/forum/index.php?topic=127928.0
Clearly visible on the index page
That guide contains duff information. It needs to be corrected.
The GM part numbers referenced and the photo are all correct. However:
APEC KIT315 contains 74mm pins / 48mm cross springs and is for 12mm solid discs. Not for 20mm vented as implied in the guide.
APEC KIT310 contains 63mm pins and is not for a Vauxhall Omega B/B2 of any description.
Mintex MBA 1271 kit contains 74mm pins / 48mm cross springs and is for 12mm solid discs. Not for 20mm vented as implied in the guide.
Mintex MBA 1266 kit contains 63mm pins is is not for a Vauxhall Omega B/B2 of any description. Seems to be for some Astra F & Saab 9-3
The mistake for APEC seems to originate from the APEC catalogue for Vauxhall. KIT315 is stated to be for Omega vented rear discs. And KIT310 for solid. However the same mistake is not there for Opel. Only KIT315 is mentioned for Opel and nothing is listed for vented.
On the other hand the Mintex catalogue clearly states that MBA 1271 is for solid discs on Vauxhall/Opel and nothing is listed for vented at all. Kit MBA 1266 is not listed as being used on any Omega B.
I don't know of any 3rd party fitting kits that actually fit vented rears. And GM never sold the vented parts as a kit or supplied them with the pads. Only the sold disc kit came with the pads. The vented parts don't seem to have been used on any other Opel/Vauxhall/Cadillac etc. I suspect that there might be something suitable from an older BMW or Merc, Autovaux were apparently able to get alternatives from somewhere but that was several years ago.
New GM 56mm cross springs for vented are no longer available anywhere, including the USA probably, since around 2013. That said I've been told that 86mm pins could still be had a couple of years ago, so it's worth a shot to see can a dealer get some on special order.
The bottom line is that you need to clean and repair the vented pins and cross-springs if at all possible. One of the German forums recommends adapting a round nail to substitute for a pin.