I saw a picture of the mortar kit on rexcrystal's website a few years ago, it consisted of a shorter blade for the harder pointed mortar, and a longer blade to cut out the brick when the pointing was cut through. I asked a fein rep about it shortly after I saw it online, and they told me it weren't Fein accesories, I tried looking it up recently, but can't find anymore pictures of it (it isn't on the site anymore where I spotted it earlier).
I was interested in it at the time, because the arbortech allsaw I have doesn't work efficiently on harder mortar types, (it's a bit improved since the introduction of the black colored blades, but still not ideal).
@ tallgras
The previous model supercut also came with an adapter for the multimaster blades (the thing you need to use for the sanding pad).
As far as I know there don't exist such attachments for the multimaster. (they do for removing grout between tiles, but the ones specific for the supercut work better for this)