For reasons that are not completely understood, there is a two-year gap in the record of FBG as a company and any projects they may have executed. Although current research has reassigned one known project to this gap and located another minor one, it remains a period of empty spaces.
Other than these two projects, extensive searching has turned up no mention of Frank Gilbreth, the FBG companies or any related project between the Boston Dispensary addition (likely completed in early 1899) and the Lowell Labs, begun in June 1902. This includes continual searching of era newspapers and the many construction/engineering journals, but is of course limited to what little has been published and the material that has been digitized.
Although Frank Gilbreth had other areas of interest such as the Weymouth quarry and patent licensing, it seems unlikely that he executed no other construction work in this two-year span, especially as he was given the major MIT commission based on his reputation.
Obviously, researchers with access to local Boston resources that have not been readily digitized and made accessible are invited to explore this puzzling gap.