Recently I have acquired a few older GE incandescent reflector lamps made at the
Ohio Lamp Plant, and notice that their date codes do not appear to fit the employed for other non-reflector general service incandescent plants -
see here. I wonder if any other forum members might know how this works?
To try and crack the code, in addition to my own lamps I had a look at a few dozen other GE reflector lamps whose etches are clearly visible on photos of this forum, and other lamps listed for sale on Ebay. This reveals a certain system.
Ohio reflector lamps made since the 1970s almost always have two letters printed at the left of the etch. This always begins with an A. Therefore in the same way that W=Winchester, S=St.Louis, M=Mattoon and Z=Oakland for A-line lamps, I presume that A=Ohio. The second letter is variable. I have observed the following: AA, AB, AC, AD, AF, AH, AJ, AK, AM, AN and AW. There appears to be a correlation between the second letter and the type of lamp. For instance all of the AA, AB, AC, AD, AM and AN were always R30, ER30 or BR30 lamps. AF, AH, AJ, AK were always R40, ER40 or BR40 (and AK always high wattage or IR types). AM and AW were always R20, the former always having C-9 filaments the latter always CC-6. Therefore I suspect that the second letter may refer to the production line within the factory.
At the right hand side of the etch is a number from 0 to 9, which is sometimes underlined. This would appear to fit the same system as A-line lamps, and probably indicates the last digit of the decade of production. Underscore probably means the first 6 months of the year, and no underscore the second 6 months.
Sometimes there are dots on both the left and right hand side of the etch. The quantity seems to vary between 0 and 3 dots on each side. I think this perhaps indicates the month code. For
A-Line codes the month is denoted by a series of dots and stars in a particular sequence, however on all but two Ohio reflector lamps, no stars have been observed. Perhaps at a certain time the system was changed to come into alignment with the A-line plants? Until very recently, I had only seen lamps with a maximum of 5 dots, and often there are no dots at all. So I therefore presumed that perhaps 5 dots would mean January or July, 4 dots February or August, with one dot being removed per month until none are left, to indicate June or December (the first or second half of the year being denoted by the year underscore). However I have seen one single example of a lamp with 6 dots. So perhaps they began with 6 and worked down to 1 - but that would not explain why some lamps have no dots.
Lastly, underneath the etch there are sometimes dashes. Four different combinations have been observed here, consisting of zero, one, two or three dashes. Although it would be unusual to indicate the week of production on a high volume lamp type such as this, I cannot think of anything else that this might denote, and the four different combinations would also match with the concept of a weekly code. Perhaps with 3 dashes for the first week, two for the second, one for the third and none for the last week of each month. It is notable however that the dashes are always symmetrical. In case of two dashes, then the centre dash is always missing. So the system did not work as in other factories where a new rubber stamp was issued at the start of each month, and each week one extra dash was cut out. Otherwise they would have required a new stamp in the 3rd week so as to be able to place the single remaining dash in the centre position. Perhaps Ohio had more complex stamps allowing them to add and remove individual dots and dashes within each stamp.
Anyway, in case anyone else is curious as to how to date older GE incandescent reflector lamps or has examples of these, I would be interested to learn more and perhaps we might then be able to date these kind of lamps with more certainty!
