GX! yes, approval of 485. see explanation below:

Exerted from other website:

OS155a is an "Immigrant Visa packet". This is given to those people whose green card was approved in a consular processing abroad. They do not get a physical green card straight away while they are abroad. They receive a stamp in the passport to enter the country and form OS155a to apply for a physical GC right at the port of entry when they enter US. CBP stamps it at the border and submits it to USCIS service center for the issuance of the actual GC.

what's your application type? If you are our regular EB1/2,I have no idea what this has to do with your case.

2 hypotheses were raised by others. First, this may be a message for a different person which was mailed to you instead by some weird twist of USCIS software. Second, this may be the actual approval of your I-485 in which IO or some service center clerk made a mistake while filing the "application type" field in the card production order by selecting "OS-155A" instead of "I-485" from a pulldown menu.

But anyway, this is an approval notice. Congratulations!