This has bothered me for a long time.
The M was dropped because it was redundant. It stood for "metric", which didn't apply to this high-level standard, since the standard applies to both Inch and Metric. The removal of the M doesn't impact reconfirmed inbetween releases.
The new ASME Y14.5 (2009) dimensioning and tolerancing standard was published in April, 2009. According to the American Society of Mechanical Engineers (ASME), "a consensus committee of experts revised Y14.5 for 21st-century applications." The new standard is a replacement of ASME Y14.5M-1994.
I got the above from: ASME Y14.5-2009: Dimensioning and Tolerance Standard
Hmm. Looks like you are right. Very odd to drop the M when the revision comes out. I wonder where that leave Y14.5 2004.
The "M" was first added in the update from ANSI Y14.5-1973 to ANSI Y14.5M-1983. I always assumed it signified the addition of metric dimensions to the standard, but I don't know that and don't find a reference.
So, to sum up, it went like this. But they are all the same standard. OK. Thanks.
ASME Y14.5 1973
ASME Y14.5M 1983
ASME Y14.5M 1994
ASME Y14.5 2009
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