Beautiful buckle, OO.
Thank you for sharing. I’m no expert, and obviously @TAH and @mmrogers will no doubt have some valuable insight, but I’ll be more than happy to provide my thoughts.
I agree with this, the areas of the casting at 12 and 6 o’clock look to be original.
I feel like this would be impossible to determine; as you well know, it could easily be either. The patina at the solder joints, as well as on the back of the settings, looks appropriate in contrast to the rest of the buckle. As you point out, the oval stones are obviously different, in both appearance and finish, than the teardrop stones, but the bezels look very similar on all 6. I find that interesting, but not indicative. The oval stones do sit well within the radii of the casting.
On the back of your buckle, it doesn’t appear there is evidence of an attachment bar/swivel bar, or a post, at least I don’t see any. As you know, soldered center bars are pretty common on cast buckles. Here are a few noteworthy examples…
This one, while rather crudely placed, appears to be flat with the back of the buckle, and could have been cast with the rest of the buckle, had the smith desired.
The center bars on both of these buckles of mine are soldered, and I feel both are original; neither buckle shows evidence of a pre-existing, cast-in center bar and both are raised from the back of buckle. Obviously the silversmith desired the depth this would provide. To cast a raised center bar (like these) integral to the rest of the buckle would be much more involved a process and an unnecessarily complicated one, than simply leaving the opening empty and soldering the bar on.
Although a bit small, this example, with its repurposed stones, remains as one of my favorite buckles. It is evident (especially at the bottom) that the bar was hammered flat at the ends for greater surface contact, then soldered to the back of the buckle.
I think the reasons a bar was cast integral to a buckle, or not, are anybody’s guess.
- The silversmith could have desired the depth that can only come from a raised bar. Bending and soldering one in place is much more practical than incorporating it into the casting.
- Technical prowess, or the lack of it, could be a factor as well. I don’t have the experience that many here have, but as I understand tufa or sand casting, the more “open work” areas of a casting that exist (like an integral center bar) the tougher it is to pull off; as the silver has to flow into all areas of the mold relatively quickly, both the temperature of the silver and the rate of pouring can be critical. The carved area of the mold for the center bar would generally be small and narrow, and if the silver cools too quickly at either end of the bar, it could provide less than desirable results. I’ll be curious to hear @mmrogers’ thoughts. To simplify my rambling and provide perspective, it would be easier to cast a simple, four sided window frame than it would be to include all of the supporting bars for a stained glass window inside that frame.
Integral bars are nice to see, and give a cast center bar buckle a more well thought out look, in my opinion.
Again, this one of mine is a bit small, but the center bar and the prong-centering ribs that were incorporated into it really showcase attention to detail, I feel.
I find it interesting that the bar on your buckle is placed nearer the face of the buckle, as opposed to being on or closer to the back. This would definitely provide a great amount of leather stacked up outside the front of the buckle when it’s worn, which gets a huge
from me.
It looks like your buckle has some incredible wear. The peaks of the casting look smooth and almost polished, as opposed to the rougher surfaces of the valleys and crevices. 
The pattern of your buckle does seem unique. I see elements from a few other patterns, but combined differently in yours. Some of these are present in the previously shared examples, and I see some similarities in design to this one, as well. Note how this looks like it was designed as a center bar buckle, but it’s got a swivel bar attachment point.
I think this is exceedingly difficult. @mmrogers, looking at the back of OO’s buckle, does it look to you like the surface texture of the silver seems to be radiating from the central oval? I fell asleep last night composing this lengthy response, so perhaps I’m just seeing things. Before guessing on age, I’d like to hear Mike’s thoughts concerning this.
I’m very much looking forward to the opinions of others. Thanks again for sharing, @OrbitOrange, it’s a lovely buckle that I’m sure you wear well!