The augment num is added to labh 'to get' preceded by ā before an affix beginning with ya,
As आलम्भ्यो गौः ॥ The नुम् is added before the addition of the affix, and by so doing लभ् becomes लम्भ् and thus loses its character of having a penultimate अ, and therefore by 3.1.124, we shall have ण्यत् affix, and not यत् affix by 3.1.98. Had the नुम् been added after the addition of the affix, then यत् would be added by 3.1.98. Though in both cases the form will be आलम्भ्या, the difference will be in the accent: यत् would throw the acute on the first syllable (यतोऽनावः) while the word has svarita on the final; thus instead of आल꣡म्भ्या (by 6.1.213 and 6.2.139), which यत् gives, we have आलम्भ्या॑ (तित् svar).
Why do we say 'when preceded by आ'? Observe लभ्य ॥ How do you explain अग्निष्टोम आलभ्यः ? All rules are optionally applied in the Vedas. Or आलभ्य may be explained by saying that the नुम् having been added, is again dropped.,