Unfortunately Vacuflo serial numbers are cryptic and pretty much have meaning only to HP themselves.
As Greg said, on the motor (which for this purpose is hopefully the original one) will be a code stamped in green (later) or black (earlier) ink. Sometimes there is a letter at the end. But the last two digits before the letter are the year of manufacture.
Also, you could unscrew one of your wall inlets and look on the back of it, as Vacuflo used to stamp those with a manufacture date.
Or, you could post a few pictures of your machine here and get a pretty close estimate of date. The earliest (early 1960s) Model 26s were hammertone blue. Then in the late 60s they were cream colored with blue trim and still had the old fifties style "Vacu-Flo" logo.
Sometimes in the seventies they became tan with a brown bucket and a red and gold logo near the top of the machine. The last design of the 26 was in the late 1980s, same colors (tan and brown) but with a brown logo that said "VACUFLO First in Central Cleaning Systems". They made the 26 all the way up to around 1992.
That unit (though not the most powerful by today's standards) is a TANK! As long as the installation isn't too big, and it's on a dedicated circuit, you should be very happy with the performance. An excellent work of engineering, which is why Vacuflo kept it as their go-to machine for 90+ percent of installations over the three decades it was manufactured.