The compensation of assessors and boards of assessment appeals, except in such towns as have an incorporated city within their limits, the boundaries of which city are coterminous with the boundaries of such town and wherein the compensation of such assessors and boards of assessment appeals is fixed by a charter provision of such city, may be established by any town at any annual town meeting and, when so established, shall remain the same until altered by the town. If any town neglects to fix such compensation, the selectmen of such town shall fix the amount thereof until established by such meeting.