The 's' modifier on the chassis might stand for stereo? It's not listed on the marketing materials.

Description:

None available

Specifications:

None available

Additional Details:

• 1 x Phono input Moving Coil, RCA pin jack
• Internal step-up transformer and output transformer for MC cartridges.
• 2 x line inputs (TUNER, AUX), RCA pin jack
• Outputs: XLR/cannon , 600 Ohm impedance
• Captive power cable

The Catherine pre amplifier separated the RA1561 circuit into two separate mono chassis, added a MM phono input and an additional line-level input. Also added was a REC OUT function for both balanced and un-balanced outputs.
The form factor and the marketing image doesn’t seem to change before the product is removed from the Shindo website at the end of 2004 ( at least 14 years! ) but the listed tube complement will change twice before Shindo removes all tube listings from their site in 2001.

Description, Shindo marketing materials 1990:

The specifications say nothing.
No matter how many words are used, the rich world of sound can never be conveyed. A profound impression that cannot be obtained by merely pursuing specifications.

Catherine
Intense time spent with something precious.  It is a treasure…

Description, Shindo website, 1999:

Separated into monaural units to achieve higher channel separation. "Catherine" replaces our mid-range pre amplifier RA1561.
Purity and natural presence are the words to describe this refined amp.

Description, Shindo website, 2000-2004:

This is an upgraded model of our medium-class preamplifier RA1561. It features a superior mono configuration with excellent channel separation, and further evolved components and circuitry faithfully reproduce the natural and subtle nuances of music. It faithfully reproduces the brilliance of 1960s analog, as well as modern records and CDs. This year, through the pursuit of advanced circuitry, it will lead you to a new world of 感動 (deep emotional impact) with an even more transparent and refreshingly expansive soundstage.

Specifications, Shindo marketing materials 1990:

• Phono MC Input, 600 Ohm impedance, built-in MC transformer
• Phono MM Input, 47 KOhm impedance
• Phono inputs switchable
• Output: 600 Ohm, built-in output transformer (balanced output possible)
• Signal to Noise: 80dB
• Power Consumption: 30W (per unit)
• Vacuum Tube Complement:
1 x ECC83
1 x 6072A
1 x 6x4
• Dimensions: 440(W) x 70(H) x 230(D) mm

Specifications, Shindo website, 1999:

• Phono Input - Switchable
• Phono MC (RCA Pin Jack), Input Sensitivity/Impedance：0.2mV/2-100Ω
• Phono MM (RCA Pin Jack) Input Sensitivity/Impedance：3mV/100KΩ
• CD. FM. AUX. TV Input (RCA Pin Jacks)
• Output (Cannon Jack/XLR) Impedance 8V/600Ω
• Recout (RCA Pin Jack)
• Phono Max. Input Voltage：1,200mV
• Total Harmonic Distortion：0.01%
• Signal to Noise Ratio：119dB.
• Frequency Responce：20 - 20,000Hz
• Power Requirements：AC 117V, 220V - 230V, 50/60Hz.
• Power Consumption：40W
• Dimensions：W 440×H 75×D 265 mm.
• Weight：10Kg (Stereo Pair)
• Tube complement：
1 x E83F
1 x ECC82
2 x 6BM8
1 x 6X4

Additional Details:

• 3 x line inputs (TV, Tuner, AUX)
• All inputs are RCA pin jacks
• Un-balanced REC OUT output, on-off switchable via front panel
• Balanced XLR/cannon jack output, 600 Ohm impedance