Sorry for not responding. I did not see your post.
Mason bees are solitary bees that can be kept in community type settings, by providing nesting sites. They are super pollinators that specialize on fruit trees and early spring flowers. They produce no honey crop for beekeepers and have limited life spans. Many backyard gardeners love these mason bees since they get pollination without the use of a full size beehive and can be kept in crowded residential areas.
Do a few google searches. (Keeping mason bees, mason bee homes, etc.) There are some books on the subject also.
Some dismiss using these as they suggest that wasps and such will sting through the side. Which is as big an urban legend as they get. But it helps some sell blocks and other "claimed" better products.
In nature, wasps do not drill into the mason tubes from adjacent holes in wood. They wait for the mason to leave the hole, hanging out nearby, then go in and lay their egg in the tube. This is how they operate. Seen it many times. They could of never survived drilling in through adjacent holes. They evolved by going straight in the tubes.
So you have all these fancy blocks and wild claims of other products that they keep predatory wasps from damaging the masons. And all the while, they will walk right in the tubes as they have been doing for eons. Some even go as far to suggest when you find a small hole in the side of the tube it was where a wasp drilled through to lay eggs. Truth is, this is where the little new wasp ate it's way OUT.
If you are considering masons, the one thing I suggest is paper inserts for whatever product you eventually do use. And always providing new nesting tubes with paper every year. These cardboard tubes are an easy system for that.