In 1935 Pennsylvania and Connecticut passed laws requiring electric light type turn signals on larger trucks. This began the era of the "arrow type" turn signal. Since the requirement for these lights was state by state you may wish to do a little research on the laws for the state in which  you will register your collector truck. As an example, California, my home state did not require electric turn signals until Jan. 1, 1955. Many of the western states encouraged the use of semaphore arms prior to requiring illuminated turn signals. Signal Stat was an early producer of the arrow type signal but there first efforsts are very hard to restore. The lenses used were vacumn formed thin plastic and are all failed now. The front assembly was also riveted and a real pain to work with. Some time later, I am guessing post WWII, Signal Stat went to a molded acrylic amber lense that was much nicer and these  sometimes survive. If you have all good pieces the only difficult portion to restore is the face plate up under the eyebrow.  When it is corroded it is nearly impossible to fill and sand out. Study any lamp you are going to try to restore, carefully, as this is the one area I have never been successful in repairing. About 1950, maybe late 1949, Signal Stat brought out their models 7 and 8 acrylic lensed drum style lights. These were very  well made and proved so popular they overwelmed the competition. While there were 15 or so truck light manufacturers around in 1951 the Signal  Stat  drum style lights garnered nearly 75% of the total market. Since these lamps were so popular and so well made most '50s truck restorers seek them to enhance their restoration efforts. 1953 was the last year arrow style turn signals were offered in most catalogs.   A bit later Signal Stat brought out a second and nearly identical appearing lamp line numbered 17 and 18. These lights were  about 3-3/4" in diameter and intended for intrastate use. The earlier 7's and 8's were about 4-1/8' in diameter and complied with ICC regulations for interstate trucking.  The smaller lights are fairly easy to get replacement lenses for the larger ones much more difficult. These lights tend to rust out around the mounting boss and the bulb mounting bracket tends to rust away. If you shake the lamp and you hear lots of clunking or rust moving around it is going to be a tough restoration at best. It is not uncommon for the mounting stud to be broken when older lights are removed from junk trucks. I have made a replacement tool and can replace them but I doubt if there is anyone else in the country that can. About 1960 Signal Stat added the Acrystat line to there product array. These are commonly found at old car swapmeets with the vendors thinking they actually have something valuable. Guess again! I think Signal Stat still offers these and if they don't the Chinese have copied them and they are real cheap if you look in the right store.

A little background on the person writing this info. I have restored antique truck lights for about 15 years now and have accumulated the only decent collection of early truck light catalogs I have ever seen. Since I have a fair knowledge of the lights I have lectured at ATHS (American Truck Historical Society) National Conventions on the subject and written magazine articles with far more details then offered above. If you don't belong to an antique truck club you may wish to look at either of these two organizations.   American Truck Historical Society (ATHS) and Antique Truck Club of America  (ATCA)