I got it from
Suruga-ya. They are usually pretty resourceful on catalog numbers for rare stuff.
Yes, it doesn't look like a usual catalog number, consisting of only words and being printed on the back of the disc, but it's not entirely uncommon. For example
Otogi OST is like that.
Though, I have to agree, it's kind of a grey area, and it is not entirely clear if this should be considered a true catalog number. But when there's nothing better to identify the disc by, this might be a good option.
In general, the normal looking cat numbers are often printed on the front and back of the disc, though sometimes they are only on the back, and sometimes they are only on the back and they look strange. LAYTON-VS-GYAKUTAN is just one of these cases.