Well, I did some more research on Mosolov and his symphonies and even if the picture does not fully clarify, a few more details have come up nevertheless. First, I realized that a while ago, Colin / Dundonnell also tried to compile a catalogue of Mosolov's orchestral music:
http://artmusic.smfforfree.com/index.php/topic,1459.0.htmlAs the three different lists of the symphonies demonstrate, it's really very difficult. At present, I believe the one compiled by Northern Flowers to be most reliable. If so, we should maybe consider the E Major symphony they recorded as No. 1 (with some care, of course).
None of these catalogues lists the "Antireligious Symphony", but this is certainly because it was only recently rediscovered. Reading the description of the German radio website I linked above, it gradually dawned me that I may even have this piece. Checking things, I found out that indeed, there is a piece in my collection just labeled "Symphonic Poem on Words by Mayakovsky and Sharov for Soprano, Mezzo Soprano, Choir and Orchestra" from 1931 which has to be the "Antireligious Symphony", even more so since it is performed by the forces the article suggests (Orpheus RSO / Kondrashev). It happens to be available online:
https://classical-music-online.net/en/production/178581It used to be on youtube but the link I have no longer works.