According to most sources, all 61's had Lanthanum glass. I'm not sure about the first two digits of the serial number being the production year. While this is true with most FSU lenses, Fed followed its own numbering scheme and even reset serial numbers from time to time. Wolves' 90-marked panda 61 would support this. Panda 61's just weren't produced that late. Even late-model Fed-3s, which stopped production in 1980, shipped with the 61LD lens.