After studying date codes of pre 1970 GE fluorescent lamps and from browsing photos on Lighting Gallery, I noticed there is an absence of GE fluorescent lamps made in 1952, '53, '57, '58, and '62. Is this a mere coincidence, or has GE made lamps in other years with an incorrect date code (e.g. a 1956 dated lamp made in 1957)?

Thanks for any info in advance!