Casing made of red plastic instead of the usual black. Probably available only in Japan.
Designed for use with the KS-02 Keyboard. Content seems to be the same as RO-610K.
When I won this pack on eBay, the seller seemed reluctant to actually send it to me, and required some persuasion. When it arrived, I discovered why - it was missing its conductive foam strip and metal latch (neither was mentioned in the listing). I cannibalised a spare RO-551 to make this into a fully-working, complete ROM pack.