I don't recall any years that did or did not have a drain plug in them. I worked in the industry, and sometimes we'd get cores in that did not have a drain plug in them, these were usually units that had been rebuild in the past. I was over at the plant Tuesday, and had a quick look at a few TF converters, and they all had drain plugs in them.
I think the unit that has no drain plug has been rebuilt in the past. Is it a used unit you've bought? If it is, unless you know everything about it, I would hesitate to use it. Over the years I've seen many "rebuilt units" that have never been apart, just nicely painted. A converter is a sealed unit, and unless you cut it open, you never know what the internals look like. The bearings and washers maybe ready to expire, and the roller one-way clutch may need to be tended to. When we rebuilt converters these internal items were replaced.
IMHO the best use for a used converter is a door stop.
