hi dave,

it reminds me of a sort of binary encoding...
0= no missing pulse 0000
1= 1 missing pulse in the "ones" position 0001
2= 1 missing pulse in the "twos" position 0010
3= ones missing and twos missing 0011
4= 1 missing pulse in the fours position 0100
5= ones and fours missing 1001

it would help to see all the way up to 9
do you also need to decode other letter/function key/enter/backspace etc. etc. characters??

I don't recognize the protocol off hand but then I am no protocol expert.

perhaps some sort of digital logic analyzer that has a good interpreter behind it would help.
something like this... https://www.saleae.com
I have never used the saleae but I know it comes highly regarded for it's price.

good luck