There is no grey area.
This is not true. Substantially not true.
I don't mean to be rude, but not only is this also not true but its very misleading, if you purchase a RIF, you are only able to convert it (relevantly speaking from IF to RIF in terms of manufacturing a RIF) if you are 'still' eligible to purchase a RIF.
If I buy a RIF when I am eligible, and if I at some point become ineligible (widely accepted to mean having not played a any insured UK airsoft site for a period of 12 months) I would lose my eligibility to also modify that article or any other article (a'la manufacturing a RIF) until my eligibility to purchase was restored.
This is still true of an IF - buying (or eligibility to) a RIF doesn't give you carte blanche to convert your old IFs into RIFs.
And in fact manufacturing is a far more dangerous offence than sale because sale places onus on retailers, whereas manufacturing attacks the little guy.
If a person is eligible to purchase a RIF, they are eligible to "manufacture a RIF". This is not grey, it's black and white.
In fact, they are eligible to manufacture a RIF from parts they do not own - on behalf (if acting without remuneration/contract) of a person ineligible to do the same.
I repeat, there is no grey area.
/notatroll