The PDP-11 computer bus, the predecessor of the more modern QBUS.
It was also used in the Eastern bloc (SMEP, JPR 12R, ...), but its physical connection is incompatible.
The L at the end indicates an inverted signal ("active low"). Some schemes (JPR 12R, etc.) use the prefix "BUS" for the UNIBUS bus, i.e. eg "BUS A 05 L" is "A 05 L", somewhere "B" is also used ("B INIT L" etc.). In modern diagrams created in CAD, you can also find inversion markings at the beginning instead of L at the end - e.g. /A05.
5.18" (13.5 cm) x 4.68" (11.88 cm) card with two side connectors (A,B). The second letter indicates the pin (A..V), the third number in the sequence indicates the side of the card.
The UBM (universal interface block, such a "backplane") is unified and consists of four positions (R1 - R4) for three-connector (K1, K2, K3) boards. And the first and last positions are only for double connectors (so-called 2/3 boards), if the bus goes to and from the next UBM (K1). So it is realistically possible to install three-connector (so-called 3/3 boards) only in the two middle positions. We are talking about general boards with a common bus, such as ASAD, PAD, QASAD, etc. Most of the other specialists had to have their own "BM" - bus boards. E.g. older tape drive controller, cassette disk controller, etc. These could not be inserted into standard UBM.