It is indeed very common in boats too small to sleep more than two persons without this feature.
No clever mechanisms are required, just sockets on the outboatd ceiling that take "hooks" (actually plates) mounted on the underside of the
outboard end of the table. These "hooks" hold the table in position fore'n'aft and thwartships when the table is up in normal position, the weight of the table being taken on the inboard end by a leg hinged to the underside of it. When down, the table lies on "cleats" (strips of wood) on the table side of the settees. Simply
lift the table and drop it into the particular position you want. The "squabs" (cushions for the backrest are dimensioned to just fill the table top, thus affording a complete matress when the dinette is in "bed" mode. TrentePieds' setup if 6'6" x 4' when in "bed" mode, basically the size of a shoreside "long double".
But there is an old
rule for small boats, say <30Ft, "Sleep two, seat four, drink six" and crowding a 30 footer with four people by relying on a "convertible" dinette is not really a satisfactory arrangement if you wish to remain civilized. The reason is that a "double" is only useful while moored. When underway and heeled, let alone rolling with the seas, sleeping on such an arrangement is downright dangerous, and no sensible
skipper would countenance it.
The real merit of this arrangement is not in its convertibility, which should considered an "emergency" feature only. The merit is in getting people out of the fore'n'aft
gangway when the boat is being worked and permitting unimpeded
passage through the length of the boat even with people using the table. That luxury is not available if the table is mounted on the centreline as a narrow plank having dropleaves to make it useful.
With the "offset" dinette, access to the stowage in the dinette seats is, admittedly, a little more difficult, but the additional difficulty is mitigated by having multiple seat
cushions, each no wider than a man's bum, and an access
hatch under each cushion. This small additional difficulty in getting to the stowage is richly recompensed by getting idle bodies out of the centre of the fore'n'aft
passage.
TrentePieds