Pylon questions

Overview

Currently outstanding questions about UK electricity pylons.

Some of the tower type designations (e.g. DTU, STX) remain undefined: what do they each mean? Moreover, what is the meaning of DTV 45° (0–5° entry)? What does the 45° indicate if not entry angle? Also, why did National Grid change this to DTU45?
Is there any vague consensus as to the definition of “crossarm”? Is there one crossarm per cable or one per rank? Depending on where you look, it seems that it can be either, so what do the “one-per-rank” folk call the arms on single-circuit towers (that are only on one side of the tower)?
Is there any definitive, conclusive information on the “(c)” and “(m)” suffixes, especially with regards to L4(m) versus L6(m)? There seems to be contradiction as to which one denotes nearest metric sizes and which one indicates a redesign, and the exact manner of the redesign.

What is the advantage of using paired insulator strings? They are common in countries such as Germany, but seldom seen in Britain (mostly commonly on the line towers of some L6 sections, such as in the Wymondley area).

Why does Britain use shorter earthwire peaks (and thus lower earthwire shade) than say Germany?
What are the criteria for requiring an earthwire? For example, most 132 kV steel tower lines have one, but a few lines in Scotland do not. “Trident” wood pole 132 kV lines do not either, just as how 11 and 33 kV do not (even when 33 kV is on steel towers).

Where does the “PB” designation come from, and was this type solely a Painter Brothers product from design to fabrication?

All known gantries seem to be SFX. What does “X” stand for, and was there also a standard SF tower? Does the X relate to that in DJ/DJX and ST/STX?

Was there ever an imperial L4 pre-dating the metric L4(m)? That is to say, does the “m” indicate that L4 was metric from the start, or does it denote a metric adaptation akin to L6(m)?