Seems you're starting to get into these now Boyd 
 
 
Lets start with block castings.
The numbers E1299A that appear on quite a large number of cylinders with EITHER hex-head or slotted coreplugs are just a part number.
Going by my data on these so far (and please dont think that is "the bible" in any way shape or form) BUT from 1950 onwards the vast majority of 102's cylinders dont  seem to have these cast onto the cylinder. Before 50 a lot of them do.
So we can agree that the powerhead at least dates from post 1950. Any other numbers that may or may not be there i have an idea they might be a batch number from a particular mold. Once the mold gets dinged and bashed beyond any further use, another mold is made and (possibly) the numbers/letters might get changed too. Another way of identifying who made it and when from a quality control point of view.There must have been quite a few molds in use all at once to produce the units required to meet the orders. So, no the numbers dont really tell me much for now!
Pumphousings.
As you're aware, around 1950 saw the introduction of the "sub ejector" pumphousing which this particular 102 has had it's outlet unceremoniously hacked off to allow the earlier swept back prop to be used. So, going by the pics this ties in with what we know already.
Gearboxes.
Oil nipples in these 102 boxes appear to be from most of the PRE 1950 motors. With the introduction of the sub ejector saw the discontinuation of oil nipples from the original 2 nipples to 1 then finally NONE at all. (this 102 has 2 nipples which would put it at pre 1950, so more than likely a gearbox swap at some point in time by a previous owner)
The correct prop for a 102 of this age is normally (but not always) a "bowtie" and NOT a swept back SD type, so something not quite right here.
"In the world" covers came before "For the world" covers, and as for a date i think it's a little bit later than 53 (nearer 56 i think)
The carb is definitely from a later stock, as is the fuel line. (bowl position will depend on the type of boat this motor is used on) Some owners prefer bowls closest the cylinder with less chance of fouling anything when steering HARD to starboard.
Choke shutters on that particular type of shroud dont normally appear till well into the 60's when the Wipac ignition takes over from the Villiers.
So another swap at some point in time. (seagull patent normally for this time period with no choke)
Transom bracket we know is from a 40 series motor.
Anything made during  WW2 would have had a short waterjacket cylinder, (42-46 dont forget) this one has a long waterjacket (and before you ask, NO the short waterjacket cylinder WILL NOT fit onto a later crankcase and vice versa) I've tried, it just doesn't fit anything like close enough as the stud hole centres are completely different. So very definitely 100% after WW2 with this one i'm afraid. The armed forces these days tend to use an outboard with some guts to it (evinrude,yamaha,suzuki,mercury etc.) and probably quite a bit bigger than an old seagull, but you never can tell. Back in the 50's things might have been very different.There might be a distinct possibility that this 102 might have been owned by the armed forces, but i doubt it somehow.
The green paint is absolutely dreadful i have to say, but not that uncommon over here in blighty. I have seen quite a few seagulls (40's centuries and 102's) painted in this exact same colour scheme with a trade mark union jack sticker on the tank. (more than likely from an ebay seller trying to make a quick buck on the side) 
All things aside, a huge amount of 102's that we see today will have had some remedial work done to them in order to keep them going. This it seems is commonplace. Dont get me wrong, they are tough 'ol birds, but eventually something will break or fall off, and you have to decide on how "authentic" you want to keep things. This 102 falls into that category.
I'm just fussy, i like my 102's to be as close as possible as when they left the factory all those years ago. Not always possible i know, but that's half the challenge with a seagull. The other half is keeping it going reliably.
So, yeah.
It's a bitsa in the kindest possible way of putting it, but i am a hard task master after all.
(but still worth a bid or two for the spare parts alone, or if you're brave enough bring it back to how it should be)
Enough of my ramblings
Jon