It is likely that in the later period the vaiShNavas were ignorant on why the name pA~ncharatra came into being. This ignorance of the meaning of the name is plainly seen in the explanation provided by the prashna saMhitA and padma saMhitA of pA~ncharAtra, and the nAradIya pA~ncharAtra has only a vague memory regarding the cause of the name. The kashmirAcharyas of mahAbala paNDita’s school and vidyAraNya yati cite a number of formulae of pA~ncharAtra vaiShNavism. But the kashmirAcharyas clearly explain the etymology by citing the vihagendra saMhitA and the sanatkumAra saMhitA. viShNu in course of the five night rite, in which puruSha nArAyaNa was invoked, lectured details on the pA~ncharAtric mantra shAstra to various recipients. As per the holy sanatkumAra saMhitA, which only survives in fragment the recipients of the lectures on the 5 nights were rudra, prajApati, indra, the R^iShis and bR^ihaspati. This knowledge was collected by the chitrashikhaNDins and provided as the vaiShNavAgamas. The same text mentions that viShNu also explained this knowledge (probably implying kR^iShNa) directly to the sAttvata heroes like sAtyaki, akrUra and kR^itavarman which came to be the sAttvata branch of pA~ncharatra. The vihagendra saMhitA states that the five recipients were ananta (=nAga), garuDa (=vihagendra), viShvaksena, rudra and prajApati, of which the vihagendra saMhitA is the collection of the lectures to garuDa. The Ishvara saMhitA mentions that the five recipients (at least in the R^iShi class) were brAhmaNas of the clans : shANDilya, aupagAyana, maunjAyana, kaushika and bhAradvAja. In addition to pa~ncharAtra the hayagrIva tantra remembers an independent tradition of tantric vaiShNavism termed saptarAtra, which was parallel to pA~ncharAtra, and contained the trailokyamohana viShNu tantra and hayagrIva tantra amongst others.
The sanatkumAra saMhitA is clearly divided into the 5 sections called rAtras with ten-eleven chapters in each, with each corresponding to the lecture of a night between viShNu and the recieving deity or sage (of them the bR^ihaspati ratra is lost). While it has some very late corruptions and extrapolations potentially attributable to the shrIvaiShNavas or mAdhvas, the sanatkumAra saMhitA by its content and memory of the actual origin of the term is close to the source. The sanatkumAra saMhitA assumes the coeaval presence of a text called nArada saMhitA (in part a potential precursor of the nAradIya pa~ncharAtra), mahendra saMhitA and padmodbhava saMhitA and the vihagendra saMhitA suggesting that these were composed together as a unit. The middle two could be internal citations to indra rAtra and prajApati rAtra (or more complete versions thereof).
The contents of the sanatkumAra saMhitA fragment as we know it are pretty remarkable. The first section, the rAtra of rudra: 1) yAgas to deities: agnI yAga, brahma yAga, vaishravaNa yAga, yAga for pacifying the vinAyaka and the uchChiShTa ulka, shrI/lakShmI oblations, the famed yaga to worship dhUrta skanda, saura prayogaM, rudra yAga, yaga to pacify various shaktis, sacrifice for yama, indra yAga, viShNu yAga, yaga for ashvins, worship of kAmadeva, soma offering. gAyatrIs for these deities and installation of their idols. 2) mantra-lakShaNaM 3) great nArayaNa mantras and gAyatrI. mAhAsudarshana mantra prayoga, the vaiShNava shmashAna prayogaM to capture and deploy yakShas and yakShiNis, rAkShasa shmashAna prayoga for sending rAkShasas aided with sudarshana, garuDa mantra prayogas, sarpa prayogas 4) invocation of 1008 gaNas of viShNu under viShvaksena, 5) harinivedana 6) flowers for viShNu worship 7) leaves for viShNu worship 8) bathing rites for idols 9) public festivals, balis and related rites 10) construction of yantras of viShNu.
The detailed tantric agnimukhaM yAgas with 4 fires for nArAyaNa and his vyUhas is provided in the brahma rAtra (this as far as my sources known is only available with some vaiShNava brAhmaNas in shrIrangaM as a manuscript).
Most interestingly the sanatkumAra saMhitA gives two forms of upanayanaM, the vedic form for dvijas and tantric agnimukhaM version for women and shUdras. On recieving that they may perform several rites, but the pA~ncharatric brahmin is still supposed to perform vaidika rites to various deities in addition to the internal pA~ncharatric sacrifices to vedic deities given in the rudra rAtra.