A glance at google maps streetview would suggest it was a parking by permit only/street cleaning set of signs.

Could be a stop sign, not that anyone stops at it

Thank you for reporting your concern to City of Cambridge via Commonwealth Connect. We have reviewed the information you provided our staff and will be investigating this issue further.

Thank you for reporting this issue to us. We are pleased to report back to you that this issue is now resolved.

The sign is still on the ground. :-( (it's a parking restriction sign).