Here is the important text from a thread I found at http://rec.games.pinball.narkive.com/4UsCL6OW/has-anyone-created-a-new-silverball-mania-rom written by Seymour Shadow (I can't find a Pinsider by that name). I do not know enough about ROMs to know exactly if this is all that is needed, but here it is.
"u1 offset a8: change 8d 1c 8d to: 20 02 14
Compared code for the top rollovers (that can spot a letter - they will
still do so) to the code for the horseshoe lane - one subroutine does it
all. branch over subroutine and adjust checksum
20 02 is bra 2 steps forward.... 14 is the checksum correction.
Now that I think of it, the patch will crash if you have the dip set to
award 1 letter instead of 2."
correction for that:
offset A6: 01 01 20 02 3e
01 is NOP
3e is the new corrected checksum"
if anyone tries this let me know!