Bronze internal split sleeve, so it is economical solution for many popular applications. You may need change to adapter with ceramic sleeve instead if you need high repeatability and durability and compatibility such as in instrumentation.

Female to female. Without screw nut. With 2 dust caps. Phos. For both SMF and MMF.

Senko P/N: 298-202. See specification at the 1st pdf link below.
ADAPTER TYPE: FC Oval Adapter “HD”,
Sleeve: Metal Straight Split
Key width: For APC Connector 2.05+/-0.02

Features:
• High Density Oval type
• APC type Adapter have Narrow key of 2.05mm
• Typical insertion loss under 0.05dB and Max IL of 0.20dB
• High quality and durable Ni-Plated Brass body

The packing box label shows vendor name is Senko Advanced Components, Inc with P/N: AF3A-P and pencil mark of P/N: 298-202, and the quality control certificate shows vendor name is Senko Advanced Components, Inc  with P/N: 298-202. But the JDS catalog at last pdf link below shows this P/N: AF3A-P is for Seikoh Giken. It seems Senko manufactured them, then Seikoh Giken imported and supplied them to JDS in Canada. See short description and original retail price of year 2000 at the 1st page of last pdf link below (from JDS catalog).

PHOSPHOR BRONZE internal sleeve, it would be same as 2nd pdf link below.  It can be used for both SMF and MMF. Samples are verified very good for SMF.  Phosphor Bronze Split Sleeves are an economical solution for applications where tremendous durability and repeated connect / disconnect operations are not required. Seikoh Giken USA Inc. told us:  the insertion loss performance is nearly the same good as Zr internal sleeve unless you use it too many times, for example, >500 mating times.