On my old 350 HR the 17" steel space saver would fit on the front or rear. (brembo brakes).
With the OEM set up on my old 350 (from memory), doesn't the wider rear wheel have a locating lug that fits the rear hub, but won't fit on the front hub because there are no holes there?
On my 370 I have obtained an OEM 18" alloy space saver, which fits either front or rear.
[i will dig out a piccy and specs]
Wheel - [part No.] 40300JK00B ... £149
Tyre - 145/70/R18 107M ... £65 [good luck finding one]