on Marlins came in 1985 and the manual bolt hold-open feature came in 1969 according to the Brophy reference. I have so little mechanical ability I know nothing about the modification. Here is my thought...in all seriousness. There are so darn many M60s in the world and unless you are really attached to yours without the LSHO just buy a new or good used one with the LSHO. Then sell the one you have for about half of what you will pay for a newer one. And there you go. Unless you are attached to that old rifle or do not want to spend the money that is probably a lot easier than fighting at your work bench with the modification...unless also you like to do those things. Just my 3 cents worth.
Edit: Sorry, I forgot you were talking about a Model 75...which is just a Model 60 with a shorter barrel and even shorter tube mag holding like 9-10 rounds. I have been meaning to experiment with how well a whole new action assembly from a Model 60 with LSHO would fit into a Model 75. That is one thought if you wanted to buy the parts. Has anyone out there experimented with this kind of changability? Will a 60 action assemlby with LSHO go in a Model 75 receiver?
Update addition in 2013: I have a Glenfield 75, I mounted it on a Marlin 99M1 stock, cut a hole in the stock for LSHO and used a new style Marlin 60 action assembly and t/g for the rifle. Now, I have what looks exactly like a 99M1 with an LSHO. Oh, I had to trade out the front sight also. The 75 front sight, although very similar, is too low so put on a 99M1 front sight to go with the 99M1 rear sight. No this was not cheaper than buying a 99M1. I just wanted to do it.