More info from

http://www.finseth.com/hpdata/hp42s.php
(Edited with Dieter's observations...thx)

Flags: 100, of which 0-10 and 81-99 are user and the rest system or reserved

0-4 show annunciator when set

11 auto execution

12 print double wide

13 print lower case

15-16 HPIL printer: 0)manual 1)normal 2)trace 3)trace w/stack print using the flags as a two-digit binary

21 printer enabled

22 numeric input available

23 alpha input available

24 ignore range errors

25 ignore any errors & clear

26 Clear - audio output is ignored Set - audio output is enabled

27 custom mode is active

28 radix mark: Clear is a comma, set is a period.

29 digit groupings shown: Clear - no Set - yes

30 stack lift disable

34 AGRAPH merging: 0) OR 1) duplicate "on" get turned off 2) overwrite 3) XOR

36-39 number of digits, 0-15 by using the four flags as a binary number.

40-41 display format: 0)sci 1)eng 2)fix 3)all by using the two flags as a two-digit binary

42-43 angle mode: 0)deg 1)rad 2)grad 3)rad by using the two flags as a two-digit binary

44 continuous on

45 currently solving equation

46 currently integrating

47 variable menu active

48 alpha keyboard active

49 low battery

50 set when a message is displayed line 1

51 set when a message is displayed line 2

52 program mode

53 INPUT in progress

55 printer exists

56 curve fit model: 1)linear 2)log 4)exponential 8)power

60 all \GS mode

61 logarithmic model invalid

62 exponential model invalid

63 power model invalid

65 matrix editor in use

66 auto grow matrices

68 base: 0)decimal 1)binary 7)octal 15)hexadecimal

72 local label

73 polar mode

74 real result only

75 programmable menu selected

76 matrix wrap, edge to edge

77 matrix wrap, first to last