As the Online Parish Clerk for Morvah I'll have a bash at answering this one lol!
The BT's do only go back as far as 1683
The PR's...... Baptisms start at 1650 (although there is only one entry for that year and then go to 1653) and go through to 1812, then start again at 1840 and I have them online up to 1901.
Marriages 1617-1763 then begin again from 1813 and at present I have them online up to 1837. Later dates, 1837-1910, I am in the process of transcribing.
Burials 1655-1812 with 1659 missing (as the vicar recorded "Anno 1659 the old Register is deficient") and again a gap from 1672-1686 (vicar's note on this "from the year 1672 to the year 1686 the old Register is altogether deficient. - afterwards very perfect") Then run from 1813 and I have them online up to 1910.
I have entries for most events past the early 1900's...... in some cases up to the 1950's but these I shall not be putting online.
I also have online the Church Wardens Accounts for the years 1855-1861 which can prove very useful as these also include details of who owned what properties and lands, the value, the amount of rates they had to pay and who had to pay them.
As I transcribe so I am adding to the site........ working out something new every few days at the mo!