I saw a couple of markers when driving through Waltham a few days ago and decided to walk over there today when I stopped at Walgreens. It ends up that this marker is part of series of markers that follow Henry Knox's trail from Fort Ticonderoga to Boston in 1775. These are the guns that Washington eventually deployed on the hills of Charlestown. which led to the British evacuating Boston.
Here's a bit more detail on Knox's trek from Wikipedia:
Knox was commissioned by Continental Army commander George Washington in 1775 to transport 59 cannons from captured forts on Lake Champlain, 30 from Fort Ticonderoga and 29 from Crown Point, to the army camp outside Boston to aid the war effort there against British forces.[1] They included forty-three heavy brass and iron cannons, six cohorns, eight mortars, and two howitzers.[2] Knox, using sledges pulled by teams of oxen to haul these cannons, many weighing over a ton, crossed an icy Lake George in mid-winter.[2] He proceeded to travel through rural New York and the snow-covered Berkshire Mountains of Massachusetts, finally arriving to the aid of the beleaguered Continental Army in January 1776.
This marker in Waltham says the following:
Through this place passed General Henry Knox in the Winter of 1775-1776 to deliver to General George Washington at Cambridge the train of artillery from Fort Ticonderoga used to force the British Army to evacuate Boston. Erected by the Commonwealth of Massachusetts.
Here's a map of the full trail from nysed.gov:
1927