I found the following information snippets in the booklet of the recording by Wolfgang Stockmeier for the label cpo:
The original instrumentations of the arranged works differ greatly, ranging from the trio to the orchestral concerto. For unknown reasons two of Walther’s concerto arrangements remained unfinished, those in D minor and B flat major after works by Giuseppe Torelli. The Concerto in D minor, originally scored for two concertante violins, two »violini di rinforzo«, viola, violoncello, and cembalo, lacks the second to fifth movements, in Walther’s arrangement, and the third movement is lacking in his arrangement of the Concerto in B flat major, originally a Sinfonia in D major for two violins and basso continuo.
The only further hint from the tracking was, that Concerto del Sigr. Meck in B minor had this marking: (Vivaldi ?)
Edit: I managed to get hold of an additional recording by Janós Sebestyén, booklet text by Carl de Nys, translated by Celia Skrine, dating from 1981.
Three of Walther’s transcriptions are based on concertos by Giuseppe Torelli (1658-1709), one of which is unknown in its original version.
He also took two concertos from the works of the Venetian Tommaso Albinoni (1671-1750) about whom he wrote a very favourable article in his musical dictionary. The B flat major concerto is No. 5 of Albinoni's Op. 2, but Walther transposed most of it down a tone and even set the slow movement, a heartfelt Adagio, in a lower octave. The F major concerto is Albinoni's Op. 2 No. 4, once again transposed down a tone.
The other composers whose concertos are here performed in Walthers organ versions are all much less familiar. Nothing whatsover is known about Luigi Manzia and indeed Walther himself does not mention him in his dictionary. [...]
In his dictionary Walther ascribed to Luigi Taglietti (1668-1715) various works which were in fact by his brother Giulio (1660-1718). The original of the concerto recorded here [B flat major] has however not been identified.
The original of the concerto by Giorgio Gentili (c. 1668-1710) cannot be identified either. Walther s lexicon mentions some ‘four and five part concertos’ dedicated by this Venetian composer in 1708 to the English ambassador to the ‘Serenissima’, Charles, Earl of Manchester [...]. As to Giovanni Lorenzo Gregori (1663-1745). Walther tells us that he was a violinist in the service of the Republic of Lucca and that in 1698 he had some 10-part concertos engraved there. The full sound of the B flat concerto played on this record makes clear that it is a transcription of one of these. It was probably intended as a sonata da chiesa, and with it Gregori shows himself to have been a worthy successor to the court and church musicians of the Renaissance.