The engine numbers may be specific to a certain model year in some cases. For example, my 2009 L3400DT-1 has an 1803 series engine, not a 1703. The engine displacement changed from 1.7 liter to 1.8 a few years ago, bore and stroke is different as I recall.
We can speculate all we like, until you can actually look at the tractor up close and personal, there's no way to be sure what it is. As an example, I could sell you a 3430 that belongs to anybody as long as I have pictures of it and you're willing to accept an arm's length transaction.
Like this one in Shreveport. I don't have the serial numbers, but I'll let 'er go for $8000..
Story time.. a couple years ago I was tractor shopping and saw a smoking good deal on a 2008 Deere 3520 on Kijiji (same as Craigslist for those that aren't familiar with it) Being naturally suspicious, I figured it was a scam.
According to the "seller" via email, the tractor was actually in Yellowknife, NWT, but he had advertised it in Halifax, NS. He agreed to pay for "shipping" to Halifax, the deal was to be brokered through "Ebay International". I was to send payment via Ebay to him, he would ship the tractor to me as soon as he received payment, I would have 5 days to inspect it and decide whether or not to keep it.
Ebay doesn't broker third-party transactions to the best of my knowledge, we didn't get far enough into the process to discover how I was going to get ripped off, unfortunately.
As soon as I started asking pointed questions about details like serial numbers and service history, he disappeared. All he actually had were pictures of someone else's tractor from Ebay/Craigslist/Kijiji. All those pictures would have cost me was $5200, including shipping.
Sean