Engine Number….

Doesn't sound right. Take a photo and paste on here, false numbering is usually pretty easy to spot.
Have you checked the frame number that should match.
Here's the dating info on Triumph Owners Club

https://www.tomcc.org/Triumph/FrameNumbers

Or they also carry out a dating service but that will cost you

https://www.tomcc.org/Home/Services

 

HD is a July 1970 frame or engine number and it would only have had five numbers so possibly a miss stamp or re stamp with six numbers ? a 70 engine is the first of the engines fitted with the untimed breather in the chain case, more pictures of engine and frame with frame numbers would help. A 70 bike frame is the last of the pre oil in frame bikes as well.

 

That last 2 doesn't look right, could it have been added to hide its original engine number?

 

The stamps all look good and match those of the period.
T120R is slightly lighter stamp as they mostly seem to be as this was done at an earlier time in the build process at the factory. It also has a slightly different shaped 2 - which is correct.
I agree with @almegro that the last 2 looks to be a 'mis' stamp.

 

The last number 2 looks to be different in font or depth to the other numbers.

If you excluded the last number 2 would the numbers make sense i.e. in a parts book?

Edit:
Obviously I'm late to the party but that's three of us with the same opinion
Need to read the replies first rather than responding to the question straight away

 

to me it looks like the right "2" but not smacked as hard.

but i suspect also it was an addition.

if it was a factory screw up i wonder what the story was?

 

Four. The 2 definitely looks a little suspect, along with an engine number in the half million plus range.

 

That last 2 looks as though it's been done by a different STAMP. compare the first number 2 with the second one.