Siddhaṃ and Maṅgala = Success and Luck.

As we know, after a certain period Buddhist manuscripts begin with the word siddhaṃ. It is this practice which lead to the script that developed after the Gupta Empire to be called Siddhaṃ or Siddhamātra. In Patañjali's Vyākaraṇamahābhāṣya (Great Commentary on Grammar) composed about 150 BCE, he says explains why this word was emphasised:

maṅgalārtham . māṅgalikaḥ ācāryaḥ mahataḥ śāstraughasya maṅgalārtham siddhaśabdam āditaḥ prayuṅkte . maṅgalādīni hi śāstrāṇi prathante vīrapuruṣakāṇi ca bhavanti āyuṣmatpuruṣakāṇi ca . adhyetāraḥ ca siddhārthāḥ yathā syuḥ iti . (I.6.26 - 7.2)

For the sake of good luck (maṅgala). A superstitious (māṅgalika) teacher begins his great, extensive treatise with the word "siddhaṁ" to bring good luck. For treatises with a lucky beginnings spread (far and wide) and produce men who are heroic (vīra) and long lived (āyusmat). And those who study [that treatise] will accordingly achieve their goals (siddhārtha). 

In a Buddhist context the Buddha is said to have been critical of those who were māṅgalika, but was not beyond making accommodations with the customs of superstitious lay people. See my essay Gesundheit! Making Accommodations with Custom.