That is a galvanised shackle with the galvanising mostly gone. I doubt it’s lost much strength yet but it’ll carry on rusting away. A new one isn’t too expensive.
so - this one seems a similar size and has a breaking load of 4500kg working load of 750kg
our boat has a weight of 1500kg (and prob about 2000kg when we're on it and our various gear) - and the Menai Straits has some very strong tides/winds at times
would this be an ok shackle size (as the chain takes a lot of the load?) or should I get something much stronger ?
Weight of the boat is largely irrelevant. It is the force you can apply and that is function largely of windspeed and the hold your anchor has. Just to give you an idea of the forces involved, my 5.5 tonne boat with a 30hp engine can generate around 300kg, which is equivalent to between 25-30 knots of wind. Your anchor well dug in might have an ultimate holding power of around 1000kgs - but you can't generate enough pull to achieve that.
So, that shackle is fine, although your chain looks on the heavy side (if it is 3/8" or 8mm) difficult to tell, but 6mm chain is fine. Don't change it though even if it is too big, it will just test your muscles more than necessary!
I wouldn’t get a shackle off Amazon. Go to a reputable specialist supplier. Going the next size up won’t harm providing it fits chain and anchor but 750kg would be enough providi by you manage any horrendous (and they would have to be extraordinary) snatch loads.
As mentioned the shackle will have lost no strength, or not that will impact performance on your rode. You could simply remove the existing rust, paint and use for a few months - and repeat. Shackles have a tough life, dragged through sand and mud - its amazing that the gal last at all.
Crosby are the best you can source, Tecni is as good a source for Crosby as any, in the UK, and better than any in Oz . The Titan shackles, made by CMP (The Rocna people) in China are available in many chandlers but have only 50% of the strength of the same sized Crosby shackle. CMP make a Black Pin shackle (the one pictured above is imaginatively called a Yellow Pin shackle) are theoretically close to the same strength as the Crosby shackle but have been of variable quality - but will be adequate for your rode.
The advantage of the CMP shackles are they are widely available and come in a range of sizes greater than that supplied by Crosby.
I think all shackles on the shelves are galvanised. If you anchor frequently that rust will soon wear off and it will look almost as good as new.
Another source is Van Beest from Holland, Belgium and France who make Green Pin shackles - highly regarded but only to the same strength as the Titan Yellow Pin shackles.
The WLL embossed on the shackle is commonly 1/5th or 1/6th of the ultimate tensile strength of the shackle (but is reduced by 50% if side loaded at 90 degrees). If the embossing does not contain a brand or source - don't buy it. And do not buy one from your nearest DIY stockist who will sell ostensible similar shackles defined as 'Made in China'. There are some really excellent Chinese shackles, the match of Crosby - but you need to know where in China they come from and there must be 1,000s of manufacturers - and some of them are really questionable.