I think Earl and Mike have the number right. It looks like 11037 to me in the pic. I guess I should have stopped by the cars when I was in Antonito a few weeks back...duh.
As far as the previous number, that last digit does kind of look like a 3, and in Sloans book he does say that it was 88153 before the final number. It's important to note though that Sloan's book is full of numerical errors (Kind of strange for a book specifically about cars), and some of these errors include some of the original tank numbers, but it would be kind of hard to do any typographical errors in the 88000 series, since they were sequential. Still an excellent book of course.
Anyway, good detective work guys. I've found in my travels around Alamosa, that only steel cars tend to retain numbers, which is fortunate for identification purposes since I highly doubt that they would have put numbers inside the tank!