This is an interesting question and one I have heard from others, too. I am glad you brought it up.
BF 4.3.8 actually says this: "Use the period, rather than the decimal point, when a period appears between a letter and a number." The BF example shows "A.4" which is a numeral following a letter, but the rule itself doesn't say what to do when a letter follows a numeral like your example shows -- 1.a -- S8CS1.a
The Nemeth Code, however, is clear regarding a numeral following a letter. Use the multi-purpose indicator as shown in NC Sec.177.ii.