A word (or two) about "Porcelain Signals"
   First off, what exactly is a "Porcelain Signal"? Early insulator collectors gave nicknames to various styles that shared certain traits in an effort to organize collections and classify the mutlitude of different styles. Though some style names are more utilitarian (no pun intended) others are more along the lines of "reminds me of . . ." nicknames.  "Beehive", "hoopskirt", Mickey Mouse", "pony" and many more are all examples of these nicknames. Truth of the matter is "if this looks a little like a haystack then that's what we'll call anything that looks like a haystack" was a pretty useful way to name similar insulators . However, because these are just nicknames for the shapes/images that collectors were reminded of (for the most part) nothing along the lines of "this is this kind of insulator" and "that is that kind" could ever be considered the technical and definitive final word. Basically this means, this is just my opinion of what "is" or "is not" a signal, for the sake of my own collection.
    With this in mind "What is a signal?" is a bit tough to nail down. In Jack Tod's 3rd edition Porcelain Guide book on pages 41 & 42 examples of various styles with their nicknames are shown. Here what we now call signals are referred to as hats (the term signal can be seen here, though only refers to telephone signal insulators, now the custom is to refer to all "hat" like insulators as a "signal" whether used for phone or power). Listed here are 5 style of insulators nicknamed "hat";
a) SR Hat - a narrow wire groove, side tie, skirt rest with a petticoat for telephone wire.
b) SR Hat - a wider groove, side tie, skirt rest with a petticoat for electrical wire.
c) PR Hat - a wide groove, side tie, petticoat rest for electric.
   The exception of this group being e) which has more than the single petticoat and more importantly a wire groove on the top . This, I believe,  should be placed in the "cable top" family not in the signal's. Using the similarities shared by a) thru d) I can set the follwing guidelines for what should generally be considered a signal.
1) The insulator should have a single, clearly distinct hat-like brim above the wire groove.
2) The insulator should be a side tie only.
3) The insulator should have at least one petticoat, though can be either skirt or petticoat rested (or both).
   Pretty straight forward so far. These guidelines now fairly well cover porcelain insulators listed from the U 299 straight through to the U 337A. However, within this group there are some styles to be excluded. The U 244A, 249, 252, 320, 322 and 336 have no brim therefore are not signals. Most of these are often referred to as doorknobs others as a mushroom. Also are the U 339 and 339A but these two are easy enough to see why they should be excluded.
    So far so good, but there are U#'s that fit the above guidelines that aren't in this number range. The U 340-345 and U 364-368 all qualify but due to their large size might be grouped as "large hats" or simpler "large signals". This keeps their signal class but shows them to be a smaller subgroup.
    Smaller styles that some might consider a signal are U 38, 39, 40, 104, 105, 113, 114, 117-121, 126, 179-180B. The 38, 39, 40 are already in the "pony" class and though they do have a brim and are side tied they do not have a petticoat. The 104, 105, 113 & 114 are already in the "toll" class and again have no petticoat.
    Next look at the 117-121 group. These certainly strongly resemble a traditional "hat" style, however, they have no petticoat. It's seems, like the multi-petticoats above, these should be still called a signal but made into another subgroup.  I've been in the habit of referring to these as "baby signals"
    Lastly are the U 179-180B and these are tougher to call. They are side tied, have a brim and have single petticoats. Tod shows the U 180 on page 41 and calls it a "stovepipe" and though not actually noted as a "hat", this is a "hat" nickname. My thinking would be to stick with this original term and make these a another signal subgroup, "stovepipe signals".
    Well hopefully I've not missed any U#'s and hopefully someone out there finds this useful or interesting. Feel free to let me know if you have any questions, complaints or suggestions.
1) U 299 thru U 337A
* except for U 244A, 249, 252, 320, 322, 336, 339, & 339A
A "signal" is;
2) Subgroups are -  large signals (U 340-345 & 364-368), baby signals (U 117-121) and stovepipe signals (U 170-180B).
d) Wide Hat - side tie, petticoat rest for electric.
e) Grooved Hat - same as Wide Hat but with a cable groove in the dome.