It's true that late 40's to early 50's was a long period. It was too long for just one trademark, and manufacturing characteristics did change somewhat. But the trans stamp category contains 4 subtypes corresponding to 4 separate eras. Each of these can be identified though trademark characteristics alone.
As far as manufacturing characteristics are concerned, these follow a rough pattern but are not nearly as reliable as just the trademarks. For example I just got a 16-1/2 type I transitional that has typical manufacturing characteristics for the era. But then earlier this year I got one that is atypical. Its manufacturing characteristics are much more like a late era example of an Avedis old stamp type, but it also bears the same type I transitional stamp trade mark. From this I had to conclude that since the manufacturing characteristics are much more like those of the the earlier era, then it must be a very early type I.
The manufacturing characteristics didn't change suddenly with every new trademark. They changed more gradually. So from time to time you'll find a few rare examples that resemble the next earlier or the next later era. The same thing holds true for the K's. So its good to know both the trademarks and the manufacturing characteristics that correspond to them and much as possible in order to get as accurate a fix on the date as possible.
