I was curious about First Street (AKA Read Street) in Lowell and so picked a deed at random and traced it back. On all the deeds in the chain going back to 1866, there was a reference to a plan by the Proprietors of Lowell Locks and Canals in plan book 00. I had surmised this meant either someone had failed to record the correct plan book and page or that possibly the plan (dated May 1857) predated Book 1 at the North Registry of deeds. After confirming is does, in fact, fall chronologically before the date on Plan 1, Book 1, I typed in plan book 00 just to see what would happen. I was surprised to find I can enter plan book 00, plan number 1 to 741! But as the plan dates span from Feb 1, 1906 and end on May 6, 1986, I really have no idea what these plans represent or why they are all lumped into a plan book that I can't imagine was really started as book 00.
Here's my answer:
There's an explanation in the "plans" section of the main page of our website:
. "M Plans" - Middlesex County layout plans - are available here. Enter 00000 (five zeroes) for the plan book number and then the plan number without the "M"
. Plans recorded before 1855 were located in plan books numbered 00, 02, 03, 04 and 05. We have renumbered those books 900, 902, 903, 904 and 905 for use on this application.
I suspect the plans you were finding were "M" plans which are Middlesex County road layout plans. The instructions say to use five zeroes as the book number, but it probably works with fewer than that. Regarding the second bullet point, 1855 is a crucial date because that is when this registry was created. Prior to that, all documents for this district were also recorded in Cambridge. Sometime after 1855, documents and plans for the towns in the northern district that had already been recorded in Cambridge were copied and placed in separate sets of books here in Lowell. They did not begin with number 1 for either documents or plans, because that number was used for new documents and plans recorded in 1855 going forward. So "Book 1" for both documents and plans begins with things recorded in 1855.