The change had largely gone unnoticed because of the language used (or not used) in the amendment. The law had read: "A tractor... may be operated without such registration upon any way ... between one-half mile and two miles if said tractor, trailer or truck is used exclusively for agricultural purposes and the owner thereof maintains in full force a policy of liability insurance."
The amendment reads: "Section 9 of chapter 90 ... is hereby amended by striking out, in line 18, the word “two” and inserting in place thereof the following number:- 10. "
Unless you are closely following a given bill's progress through the legislative process, sections like this one are easy to miss.
