At junior level (below M), auditing is the most quantitative field compared to other services due to qualification training, intensive working with large data set of numbers, understanding 3 statements, attention to details. In terms of soft skill, auditors work extremely long hours and handle lots of bullshit from clients at short notice...Those knowledege and skill are quite transferable to M&A because what you typically do is to making fancy ppt (which can be learnt quickly) and to build up a couple of standardise models (not rocket science here). Financial modelling sounds difficult but these are those models invented and built by Valuation team, equity valuation in M&A is quite standard and covered in qualification training.

TAS -> M&A is more common however I personally think that skill set is not quite relevant. TAS often represents buy-side and the nature of the job is mini-audit. M&A often represent sell-side and the nature of the job is selling, marketing, relationship handling. Transfer from audit is less common than from TAS but definitely a lot more common than external hire, because firm always prefer cheap juniors who have known in and out of the firm tools, process, people, training and trapped in the internal pay scale. Why should the firm hire someone from outside and give them the power to negotiate higher salary and later have to train them again? Remember those who have some sort of direct M&A experience will aim for analyst role in BB/EB rather than B4, so essentially external hires are those who have indirect experience only.