Aye weel, that didnae take long at a'
Things crystalised quite quickly for me, so to speak..............
I reckon that there was never a Thomas, only ever a Robert.
It's possible to dream up scenarios where Thomas changed his name to Robert; or Thomas died and his widow remarried his brother Robert, of which there is no evidence, but applying Occam's Razor (Gie it a Google if ye arnae familar wi 'Occam') leads me to the following conclusion.
There's an error on the 1867 marriage record resulting in Robert being shown as Thomas.
It's not generally realised that the procedure for the registration of a marriage was quite different. For a B or D the informant turned up at the office of the registrar, and, in front of the registrar or his assistant, the info was there and then entered in both registers (one was kept locally, the other was sent to Edinburgh the following January).
The informant then signed or made their mark and that was that !, unless an RCE entry was later required. Once the register entries had been signed by the registrar, an alteration or addition could only be made via an RCE entry; OK, in the first decade or so, there was some confusion over the procedure, and an addition/alteration will occasionally be seen in the margin of the register entry.
For a marriage, however, the couple obtained a "Marriage Schedule' from the registrar, - I've never been clear whether this was filled in in the presence of the registrar to the extent possible, or only later in front of the officiating minister.
Whatever, after the ceremony, somebody, often the best man trotted along to the registrar's office and passed over the completed schedule signed by the couple, the witnesses, and the minister.
The registrar then copied all the info from this Marriage Schedule (engrossed is the guid auld Scots word) into his Register Books of Marriages, and that was that.
Two possibilities for error here, - wrong info during the filling in of the schedule or an error during the transcription.......
These schedules still exist, but only in really exceptional circumstances will GROS grant access, and then, only to the signatures of the couple, which might well not help here if Robert just made his mark !, but maybe just still worth it.
For the 1868 birth of Annie, the informant is the granny Margaret SCULLION, and it shows the father as Robert.
On the 1870 birth register entry for wee Thomas I can well imagine a scenario where the registrar asks Robert "
What's the father's name?" but Robert mishears that as "
What's your father's name?" and replies accordingly, resulting in the father of wee Thomas being shown as Thomas FITZSIMMONS. Thomas/Robert makes his mark in the register.
On both these birth records the date of the marriage is shown as September 1867.
In 1874 Robert loses his wife to Phthisis Pulmonaris, aged 26. The Widower, Robert FITZSIMMONS signs the register but it is a very halting and unpracticed signature .......
Every relevant record thereafter refers to him as 'Robert'.
Oanybuddy goat a different thesis, but ?
What would help in order to go from strong suggestion to certainty would be an 1871 census entry for the family of Robert, Mary, Ann[ie] and Thomas, but I'm d****d if I can find one even turning numerous sumersaults with wildcards and considering areas other than Ayr .........
Similarly, it would help to find birth records for the family of Thomas FITZSIMMONS and Helen FITZSIMMONS MS DIVIN[E], Robert's parents, but could it be relevant that the 1867 marriage of Robert/Thomas FITZSIMMONS and Mary SCULLION was according to the Forms of the Roman Catholic Church ? (just where, I wonder, are the long promised index and images of the Scottish Catholic births, confirmation marriages, death etc.? )
And again, similarly, after 1874 I just cannot find any further record for Robert/
Thomas, such as a remarriage (the wains, Annie and Thomas, in any case were obviously looked after by grandparents and other relatives), a death record that fits, or a census record in 1881 or later.
Did Robert/
Thomas leave Scotland, going back to Ireland or elsewhere in the UK, or even further abroad, furth of Auld Scotia ?
Pheeeeewwww .................
See some genealogy puzzles, they wad burst ye!, but, to use guid West of Scotland vernacular
ibi