How do I keep this old shower pan water tight
Nothing but caulk (100% Silicone), is the hands-down best I've found.

The Wonderboard would sit (actually stand-on and rest-on before screwing down) on the Terrazzo's top and back lip in a squished-out caulking bed for drain backup flood waterproofing.

You really don't want any firmer type of gap-creating "gasket". Then, your Redguard, Thinset &amp; Tile overlap that seam. They are all that really makes everything actually waterproof.

You can then chose to either rest your tile down to &amp; on the inner and lower secondary Terrazzo lip above the pan floor or you can raise it slightly to grout a larger joint. Neither is any different in waterproofing, but not grouting means no ugly possible breakaway crack later.