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Polished brass left-hand mortise knob lockset with surface-mounted plate and screw-on knobs

Marks USA

Marks USA 91D/3 Polished Brass Left Hand Mortise Door Lock

SKU CH-1NA3-7HXF8 MPN 91D/3

A left-hand mortise lockset with classroom function, solid brass construction and a 2-1/2" backset

  • Backset distance2-1/2 in
  • Lock keyingKeyed Different
  • Main functionClassroom
  • Surface finishPolished Brass
  • Sold asBox
  • Product typeMortise Surface Mounted Lock Knob, Office Vestibule, Passage
  • Classroom function keeps outside lever locked until key unlocks it
  • Latch and rocker design operates without a deadbolt
  • Solid brass plates and cylinder resist wear and corrosion
  • Left-hand orientation fits doors hinged on the left
  • Surface-mounted screw-on knobs with swivel spindle simplify installation

Polished Brass Mortise Lockset

The Marks Metro 91D/3 is a surface-mounted mortise lockset finished in polished brass and configured for left-hand doors. It uses a classroom function that provides an office vestibule mode, allowing the latch to remain always open or always locked depending on the setting. The lockset includes a solid brass cylinder with two keys and screw-on knobs with a swivel spindle for installation.

Mechanical Operation and Durability

The lock operates entirely through mechanical action with no electronic components, so it generates no heat, noise or power draw during normal use. Solid brass plates and a brass cylinder resist corrosion and wear in commercial environments. The latch and rocker mechanism functions without a deadbolt, relying on the cylinder and key for access control from the outside.

Installation and Daily Use

A 2-1/2 inch backset and a face plate measuring 1-1/16 by 7-5/8 inches determine the mortise pocket and trim dimensions required for fitting. Surface-mounted plate assemblies with screw-on knobs simplify mounting on prepared doors. The classroom function means there is no thumb turn on the interior side, so egress is always free while exterior access remains key-controlled. Buyers needing a deadbolt or a privacy turn function should select a different lockset.

How it compares

Measured against everything we stock in Door Lock Sets & Deadbolts, at today’s prices. Recalculated every night.

Price $98.99
Pricier than about 65% of the 2,145 comparable items (median $55.99).

Specifications

Product
Brand nameMarks
Product lineMetro
Model name91D/3
Product typeMortise Surface Mounted Lock Knob, Office Vestibule, Passage Always Open OR Storeroom Always Locked, Classroom, Latch and Rocker (No Bolt) Lockset
Design styleKnobs
Size & build
Surface finishPolished Brass
Main materialBrass
Backset distance2-1/2 in
More details
Lock keyingKeyed Different
Main functionClassroom

Questions about this item

How do I install the Marks 91D/3 mortise lock body into a door prepped for a 2-1/2" backset and what step is often missed?

Mortise the lock case into the door edge so the face plate 1-1/16" x 7-5/8" sits flush; the step often missed is aligning the swivel spindle with the screw-on knobs before tightening the surface-mounted plate assemblies.

What maintenance will the solid brass cylinder and latch rocker mechanism need over time?

Lubricate the cylinder and rocker pivot annually with dry graphite; the solid brass cylinder and two keys resist corrosion, and the latch rocker has no bolt to adjust.

What is the first limitation that appears when the classroom function lockset sees heavy daily use?

The screw-on knob attachment can loosen under high-cycle operation, requiring periodic re-tightening of the surface-mounted plate assemblies.

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