Mark, this is how it looks at Mercedes here, prices in PLN:
A 180 053 19 52 SHIM (PODKLADKA KOMPENSACYJNA) 4.5 MM 79,39zł Available at the dealer
Up to engine: 980 10/50 015834 980 12/52 033133 981 10/50 000343 981 12/52 002247 983 10/50 005464 983 12/52 009136
A 130 053 03 53 THRUST PIECE (ELEMENT NACISKOWY) 4.5 MM STANDARD 145,47zł Available 72hrs (Germany)
As of engine: 980 10/50 015835 980 12/52 033134 981 10/50 000344 981 12/52 002248 983 10/50 005465 983 12/52 009137
I do not know if the shim needs to be of certain type (earlier or later) if you intend to change the rockers for the newer type (not colliding with the spring retainer). I do not know if one depends on the other.