Ernst van de Wetering’s hypothesis in direct relation with the painting,
the Baptism of the Eunuch, ca. 1630:

“Missing paintings: Although we suspect that the greater part of Rembrandt’s oeuvre has been preserved, it is still possible that many paintings have disappeared. Sometimes traces of such paintings remain, for example in written descriptions from the past, or in copies or reproduction prints. The latter is the case with the scene illustrated in fig.55 with the Baptism of the Eunuch. It is an exceptionally ambitious reproduction print (59.2 x 49.1 cm) of a painting that Rembrandt was apparently very proud of. There is probably another trace of the same painting: a rapidly painted head, done in broad strokes, of an old man bending forward.“
E. v. d. Wetering in A Life in 180 Paintings, Local World BV, 2008

“The resemblance goes further than the outer appearance of the model. It extends to the facial structure and the means used by the artist to depict it.” Gary Schwartz, A Rembrandt invention: a new Baptism of the Eunuch, 2020, p. 55.

Is the Baptism of the Eunuch by van Vliet faithfull to the painting?

“We think this is an oil sketch for the head of the apostle Philip who is baptizing the rich Moorish courtier whom he was converted to Christianity. The print (reproduced here in mirror image) was produced – without doubt commissioned by Rembrandt himself – by the graphic artist Jan G. van Vliet. It would certainly have contributed to Rembrandt’s fame.”
E. v. d. WeteringRembrandt, A Life in 180 Paintings, p. 46 Local World BV, 2008 Local.