Specs were detailed in the LE dossiers. Also, some of the playtest materials came with unit loadouts, so they exist...
I'm pretty sure all LE dossiers are available in PDF form for free, and i believe i read every single one, yet recall no Mangonel stats presented anywhere. The Mangonel may have been mentioned.
So i wonder if Catalyst had the specs behind the scenes but they weren't available for public. That, or the PDF compilations are not complete.
Or maybe they're in the playtest materials, i don't recall ever seeing those in PDF form.
http://www.sarna.net/wiki/MangonelInterestingly Sarna notes: "One Mangonel sighted in action during the Dark Age era was described as mounting Autocannons instead of Gauss Rifles." with citation needed.
Was Anastasia piloting the -3W?
http://www.sarna.net/wiki/Anastasia_Kerensky#BattleMechSarna says: "She later piloted a MNL-V1-H Mangonel that was affectionately named "Alpha"."
One of the weird Dark Age designations. The variant hasn't been canonized, so it is unclear if the designation still holds true and the 'Mech is merely lacking stats, or if the designation is basically uncanonical.
The other named Mangonel, the Copperhead, has identical designation, with very different stats. The generic Mangonels have designations MGL-V1-H and MGL-V3-H, according to Warrenborn.com.
I'm inclined to believe her Mangonel might be customized (on the account of her being an important character and having piloted customized Ryoken II), but no idea which variant it might be based on.