Yes, Ric, that's the one I downloaded: iOS 12.4.2. (I was using a shorthand version in my post.) As you say, 12.4 isn't even available. The signing server still returned an internal error, after many tries, with 12.4.2.I believe you'd have to download iOS 12.4.2 (or possibly 12.4.1). The older version without security fixes is no longer "signed" and is thus unavailable.
And, by the way, there was no way to update the iPad Pro over the air to iOS 12.4.2 – the only option available was 13.1.2.