i dont think its anything special really just one of the thousands of k2~ heads that mr Honda slammed out in the seventies could be a k1 but the oil metering jets makes it at least a k2 to me or even later not sure when they changed for them type of oil jets. its not a sandcast or a k0 just my thoughts on it .

  ko 's had alloy tops  and chrome under the springs  and they would be die-carbon gas filled not oil filled  I think, but yes they are still a good buy at that price

  @kp yes it can be  subtitled but its a bit hard to follow  even france had the sandcast and the cb750k0 before us poor britts

  yes another gold digger blows his wade ,good old fleabay  it brings out the fattest parasites. even j leno showed an interest   I love for him to keep it and pass it on to his kids would'nt that be sweet

  does not look a repair to me it looks very intentional, im wondering if the welding was done to raise the pegs and give them more ground clearance, was  it ever raced in an earlier life? you would be surprised what them old racers got up to
 my first cb750 had a nineteen inch rear wheel fitted by a previous owner who said he had used it for twisty sprints and hill climbs ,he said the bigger wheel gave him more ground clearance , just my theory

option 3    i think is you best bet,
option 4    i would forget trying to bodge a set of 341's to look like 300's because real CB750 fans will spot them a mile away,on closer inspection they are very different inside as well as outside, if you gonna use the bike on the road you may as well fit the later pipes (341) , i have done this myself with one of my bikes its your choice really on how original you want the bike as it will all reflect on the sale price anyway this always seems to happen with projects, they either run out of money or rush to get it finished and cut corners as you can see on this site some owners here are very meticulous with their sandcasts and get them right down to the last detail, bearing in mind yours is not a sandcast and will not be the same value, you have done a pukka job so far, if it was my bike i would put a set of stamped 300 pipes on it but that's me, its your call, none of my bikes are show bikes , just there to be ridden so i don't worry to much about them being 100% original
 option 5   is a none starter you would need to give me loads of money and body parts , i have a spare set of 300 pipes, i bought them a few years ago i knew they would only go up in price or they would dry up altogether, its a good thing they are still making copies, lets hope china can cash in like Vietnam 

  you could try soaking them in hot water they might be transfers.
 i bought mine from silvers i think and they are already cut to shape, not even sure if they are transfers it was that long  ago ive yet to use them as i have put them somewhere safe  i will find them one day