Well done, Gert.
The primary address range of the OS PROMS is from 0000H-01FFH. I believe the Czech replicas (one of which I think you must have) are of the issue V MK14. Could you look at address 0200H onwards to see if there is a copy (image) of the PROM code there? I would be interested to know if the address decoding modification (discussed earlier in this thread) has been incorporated into those replicas.
What was the first program you ran on your replica? 'Message', from the original manual, is often popular as you can see something happening and then have some fun trying to devise your own scrolling message without using 'K', 'M', 'V', 'W', 'X', or 'Z'.
You mentioned that you had to use 74LS257s instead of the 74LS157s you had originally. What prefix / brand were the 74LS157s which did NOT work?
I have found a UK source for original Nat-Semi branded DM74S571 PROMs, a pair of which arrived today. At almost 12 GBP for the pair they weren't especially cheap but in light of the discovery that I can not program the Tesla MH74S571 devices, I thought I had better obtain a pair of National Semiconductor 'DM' prefix devices to program up and keep as spares for my MK14.
I've asked a one-off favour from someone who believes they can program Slothie's Tesla MH74S571 PROMS, and we are awaiting the outcome from that.
For anyone else, I'm happy to program (and test) PROMs for an MK14 if they are any of the following devices:
National Semiconductor DM74S571 (but not DM74S571A)
Philips / Signetics 82S131 -or- 82S131A
AMD AM27S13 (but not 27S13A)
Obviously, they must be unused blanks because this type of PROM can not be erased and reprogrammed.
I am NOT able to program the Tesla MH74S571. I believe Slothie has a medium-term plan to make a specific programmer for those...