Hi Ray,
The "3" version of the Siltronix 90 puts out 16.27 MHz on channel 1. Matches the Mark III SSB transmitter, but not the AM version.
25 years ago Glenn Manufacturing sold a converter board (called the "326-1") that would adapt the Glenn slider to a transmitter like that. Haven't seen one of those for sale in years.
The reason you need it has to do with the crystals in the radio. Normally, you pull out a crystal and install the cable from the slider in its place. Select the channel that you plugged the slider into, and off you go. In the Mark III SSB tansmitter, the 16.27 MHz coming out of the slider is so far from your receive frequency that it doesn't cause receiver problems feeding into the tansmitter while receiving.
Trouble is, in your AM-only transmitter the crystal runs ON the same frequency as you are receiving. The slider will throw a "spot" carrier into your receiver, covering everybody up.
Years ago, we modified Pal sliders by installing a "key" transistor inside it, and a separate "keying" wire from the transmitter. The slider would only run when you keyed the mike or pressed the "Spot" switch.
Made a prototype batch of a board to take the place of the Glenn converter, and it uses a crystal to make the "-3" slider work with it. Trouble is, it's a prototype, and nowhere near ready for "prime-time". Can't really sell it as a do-it-yourself kit until a few things get refined, and a 'production' batch of them built to sell.
There is a fella named Alan who sells a slider for $150 plus shipping that's already set up for your transmitter.
tubesplus.tripod.com/tubespluscom/id8.html
Just checked the web page, and it's listed as fitting the S-23, Mark II and Mark III AM-only version.
Couldn't get him to sell me one, so I can't report how well it works from experience. Maybe you'll have better luck. And if you check out the old radio models on his "Items Wanted" page, maybe you have one of the early 40-channel radios he's looking for. You never know, he might trade out part of the sale price on a few of those. I'm gonna behave myself, and NOT tell you what he does with them.
73