Existing approved residential lots or lots validly of record with frontage on public highways or contained on certain subdivision plats filed with the County Clerk for lands rezoned to the RA-.5 Zoning District in 1989 (Local Law No. 4 of 1989). All existing approved residential lots or lots validly of record in the Town of Montgomery as of July 10, 1989, which were rezoned to the RA-.5 Zoning District in 1989 but contained the requisite frontage on any existing state, county or Town highway as of July 10, 1989, or those lots which are included on a filed subdivision plat previously filed with the County Clerk pursuant to the interim grandfather provisions of Section III of Local Law No. 4 of 1989, may be developed and improved in accord with the zoning lot area and bulk requirements of the Town of Montgomery in effect immediately prior to the adoption of Local Law No. 4 of 1989. Notwithstanding the above, all Zoning Law amendments subsequent to Local Law No. 4 of 1989 shall apply to all such lots. Additionally, no such lot shall have been nor shall be further subdivided to create any new residential lots that do not meet the minimum requirements imposed by this chapter as existing now or as, from time to time, amended, unless the subdivision plan is an approved cluster plan in accord with the Zoning Law existing at the time of the filing of the clustered subdivision plan with the Orange County Clerk.