Here's a useful process for determining interchangeability of parts:
1. go to Service Honda's parts fiche
http://www.servicehonda.com/look-up-parts/03 (other sites have this too)
2. get the part number
3. go to BikeBandit
http://www.bikebandit.com/ and search the part number
4. Click on the "Fits Machines" link and you can see what bikes use that part
>Now you know what donors you can be searching for a part!
Keep in mind that the key will almost assuredly be different if you get a whole new lock - there are sites on the net that supply many old honda keys though.
I already did this for you, here are all the bikes that use the same locking mechanism:
1972 Honda CB175K6 View schematics
1972 Honda CB350F View schematics
1972 Honda CB350K4 View schematics
1972 Honda CB450K5 View schematics
1972 Honda CB500K1 View schematics
1972 Honda CB500K1 View schematics
1972 Honda CB750K2 View schematics
1972 Honda CL175K6 View schematics
1972 Honda CL350K4 View schematics
1972 Honda CL450K5 View schematics
1972 Honda SL350K2 View schematics
1972 Honda XL250 View schematics
1973 Honda CB175K7 View schematics
1973 Honda CB350G View schematics
1973 Honda CB450K6 View schematics
1973 Honda CB500K2 View schematics
1973 Honda CB500K2 View schematics
1973 Honda CB750K3 View schematics
1973 Honda CL175K7 View schematics
1973 Honda CL350K5 View schematics
1973 Honda XL175 View schematics
1974 Honda CB200 View schematics
1974 Honda CB350F1 View schematics
1974 Honda CB360 View schematics
1974 Honda CB360G View schematics
1974 Honda CB450K7 View schematics
1974 Honda CB550 View schematics
1974 Honda CB550 View schematics
1974 Honda CB750K4 View schematics
1974 Honda CL200 View schematics
1974 Honda CL360 View schematics
1974 Honda CL450K6 View schematics
1974 Honda MT125 View schematics
1974 Honda MT125 View schematics
1974 Honda MT250 View schematics
1974 Honda XL175K1 View schematics
1974 Honda XL250K1 View schematics
1974 Honda XL350 View schematics
1975 Honda CB200T View schematics
1975 Honda CB360T View schematics
1975 Honda CB550K1 View schematics
1975 Honda CB550K1 View schematics
1975 Honda CB750K5 View schematics
1975 Honda CL360K1 View schematics
1975 Honda MT125K1 View schematics
1975 Honda MT125K1 View schematics
1975 Honda MT250K1 View schematics
1975 Honda XL175K2 View schematics
1975 Honda XL250K2 View schematics
1975 Honda XL350K1 View schematics
1976 Honda CB200T View schematics
1976 Honda CB360T View schematics
1976 Honda CB550K View schematics
1976 Honda CB550K View schematics
1976 Honda CB750K View schematics
1976 Honda MT125 View schematics
1976 Honda MT125 View schematics
1976 Honda MT250
Finally a quick funny story:
I keep my StaBil ethanol treatment under my seat...while filling up the other day I placed the keys on the airbix while adding the StaBil and gas....and then I replaced the Staybil and shut the seat, locking the keys under it! Luckily, the gas station had a coat hanger and I was able to get them out after a few minutes....