You will need to refer to BSP 518-255-401. See page 9, specifically.
Click The tel set Sl/Y leads are attached to the pairs of pins 27 through 34, row A, that are labeled "B1" & "R1"
There are also pins 35 through 38, row A, that must be strapped for as many lines (1 through 4) that are to ring as a group. These are labeled "CA" (Common Audible)
The 551, however, does not come equipped with a ring generator supply. Originally, an outboard 20B2 power unit was mounted to provide generator, a case of overkill, so later, the 118A frequency generator was introduced. It is a small device, about 2" by 3" by 4", with a short 110Vac cord, and it can be mounted on a bracket (part number 841720857) that fits in a spot in the KSU.
Since even during the time that these were in service, the brackets were made of unobtainium, the field expedient was to mount the 118A with a tie-wrap around, and just in front of the interrupter.
The generator output leads from the 118A are punched down at the lower right, on pins 49 & 50, row E, labeled RG (tip) and 105V+/- (ring).
If you want selective (non "square") ringing, you must provide a diode matrix, generally mounted outboard of the KSU, and you must convert each set for matrix ringing by removing the network capacitor form the ringing circuit.
A less elegant solution is simply to assign the ringers in the tel sets to the T&R of the two CO lines, so some phones ring on line 1 and others ring on line 2. This can accomplished in either of two ways. You can assign the Sl/Y leads at the KSU to the T&R pins of the two lines, or you can open the sets and move the R/Bk ringer leads off RT & RR on the line key to the T & R terminal screws on the key, such as T1/R1, T2/R2, etc.
