I have a dim recollection of the following short/long deck.

Alternate the short and long cards. Each short/long pair has, with the deck face down in the left hand . . .

- The upper surface (i.e. back) of any given short card shows the value (or mate) of the face of its paired long card sitting above it.

By way of example, let us say a random pair is, from top to bottom . . .

1) Upper card [long card of pair] face/bottom surface = 3D
2) Card below it [short card of pair] back/top surface = 3D

So, hold deck in LF with face/bottom toward spectator and ask them to say stop as you riffle down the index corner of the deck. The short cards click off the finger causing the card stopped at always to be a long card. When you stop, the back/top surface of short card facing in your direction will show the value of the long card above it, revealing the identity of the selected card to you.

Variants could include:

- Roughing the pairs to hide the back/top surfaces.
- Making the back/top surfaces of short cards facing in your direction not identical to the card facing the spec above it, but another indicator card (e.g., when you see the 3D, the card above facing the spec is the 3C [same value, opposite suit]; or, when you see the 3D, the card above it facing the spec is the 8C [adds to 13, opposite suit].

My memory is weak. I can make this deck up but wonder if it is/was commercially available. Thank you for any guidance.

I don't know about commercially available, but aren't you describing a (roughed) M-T deck? It'll be available unroughed.