I assume the washer came from a parts order and was not on the engine...
Browsing the EPC,I found that the gasket A6162030180 is a direct replacement of the older number A1802030980 on later type om 615,616,617.However,on om 621and early om 615 the replacement of original gasket(A1802030680) needs A6162030180+the washer(or A1802030980 +the washer)
Links to confirm:
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=115100&M=615%2E912&GA=722%2E107206&GM=716%2E002&CT=M&cat=005&SID=20&SGR=001&SGN=01
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=110116&M=621%2E918&CT=M&cat=020&SID=20&SGR=001&SGN=01
..there is no mention of the A621XXX compensating washer on engines newer than 1968....:
*001 UP TO ENGINE:
910 000002
912 10/50,20/60 000148
912 12/52,22/62 000014
913 10/50,20/60 000020
913 12/52,22/62 000002 (the very beginning of manufacture,some old stock parts used on new w115 models)
you can check this on the epc.
http://www.detali.ru/cat/oem_mb0.asp?TP=1

However,some online parts stores may list this as valid to all om61X models...
http://www.***************/search/product.aspx?sid=axsfht45m2bzy4bmrl3023r0&partnumber=6162030180

IMHO,you really do not need this washer.