If you are sure those are both decimals -- neither one is a period -- then you must follow section 177. The multipurpose indicator must be used after the decimal point to indicate that the symbol that follows it is not numeric. So the dot 5 should be between the decimal point and the following letter. The second dot 5 would precede the decimal point between the D and the 1 to indicate that the decimal 1 is not a subscript. An update to the Nemeth Code states that letters in abbreviations that are a mix of numbers and letters should be capitalized individually.