Question: Any idea what PROM is intended in this diagram is ?[![PROM](https://i.stack.imgur.com/PAQPB.png)](https://i.stack.imgur.com/PAQPB.png)

It is from the rational R1000 computer.

Edit: Its not my project, I was trying to help the Danish data museum. They are making an emulator for this computer.
And I realized I might have misunderstood their original question, but I will let this question stand, and accept the best answer.

Answer: 
Most likely a standard 4 KiBit 512 by 8 Bipolar PROM.

Going from pinout and assumed DIP 20 it's either an

* AMD 27S28 (OC output), or
* AMD 27S29 (Tristate output)

From the circuit shown I'd assume it being a [27S29](https://www.tekkna.it/open2b/var/catalog/product/files/7738.pdf). Timing wise both are the same.

Compatible types for AM27S29 (AM27S28) are

* Harris HM7649 (HM7648)
* Monolitics Memories MM53/6349 (MM53/6348)
* National DM74S472 ([74S473](https://www.digchip.com/datasheets/parts/datasheet/321/74S473-pdf.php))
* Signetics S82S147 (S82S146)
* Texas Instruments SN74S472 (SN74S473)

I wouldn't get confused by different (inverse) logical address/data numbering, as that works fine as long as the programming file is coded accordingly.
it is almost certainly a 512x8 bipolar prom.

It is probably being used as a decoder.

They were common in the late 70's and 80's before fast CMOS ones were available. They are all obsolete now although some are still available in the surplus market.

Apple used them on the Apple II disk controller and printer cards.