Nothing in the book as such I'm afraid, (besides which, the contents of said book are subject to 'Copyright'), but the process is as follows...
The Planning Controller formulates a clearance using the flight data processing system and forwards it electronically to the (VHF) Clearance Delivery Officer, or (HF) Radio Officer, in order that they (the CDO/RO) can dictate the clearance to the Crew and obtain a readback. (Alternatively, the Planner may send it directly to the Crew via ORCA/ACARS if the A/C is so equipped).
If the clearance includes any changes to the route/oceanic entry point/speed/level as specified in the FPL/Request, such changes are highlighted by the Planner in a 'Remarks' section of the clearance, which is the equivalent if you will to the 'Field 18' in a FPL.
Those versions of the clearance for delivery by VHF/HF voice are automatically converted from an abbreviated format, used by the Planners, into a plain language format to be dictated by the CDO/RO, with any 'Remarks' highlighting a change to the FPL/Request also appearing in plain language as part of the clearance.
For a change to the level from that requested, as in your case, the Planner would include "LCHG" in the clearance Remarks field. This then appears as part of the clearance, in plain language, at the CDO/RO's work station as follows...
"(Callsign) CLEARANCE WITH A LEVEL CHANGE. SHANWICK CLEARS (Callsign) TO (Dest) VIA...."
Therefore there is no RTF phraseology as such, given that the system simply converts the clearance, including any change highlighted by the Planner in the 'Remarks' field, into a plain language message, which is then dictated by the CDO/RO.
In the case of Santa Maria, I can only assume that the flight data processing system they use does not have a similar degree of functionality, but there is certainly no harm in drawing their attention to the benefits of having such.