Hi Dave!
I don't know why they are different, but it is usual for sliding calipers.
My guess as to why the L2 rubber is missing would be it was distorted or otherwise damaged, somehow, so would bind if it was fitted.
I have had this happen to me on our Subaru. I left it out and it was fine, but I'm sure it is there for a purpose!
So, and I must qualify that I am not a professional, especially as this is brakes, on my own vehicle -
I would put it back together in the way it has been for the previous 7 years, using the correct caliper slide pin grease - NOT COPPERSLIP - it will probably bind up, yes that was the Subaru too. Yes, we always used it, but something has moved on and it is no longer the stuff for the job.
Then locate the correct part on Amayama and swap it when it comes.
I recently did this for our Vellfire calipers, they came as a pack of the piston seals, sliding pin boots, bleed nipple caps, banjo bolt washers, and the little rubbers for the pins I actually wanted, for £12!! I bought new slider pins too as they were only £2.40 each!
Arrived in 10 days!
Cheers,
Nev