These post are quite advanced compared to those that I grew up with.
You are correct though today about the road number identifier.
The 5.1 will be the kilometre marker*, meaning that post is placed at every 0.1km or a 100m. I suppose this would help emergency services as well as you to identify let say your breakdown or worse accident location.
The 73* just after 5.1 I strongly suspect, is the total distance of the road that is L106.
The triangle or arrow is the direction of the countdown, meaning that in 5.1km you shall be arriving at the end of the L106 which will start/end in the town or at a junction ahead of you, on the river bank.
You may have picked up on the fact that the post is only so tall around 0.8-1m in height, and that it has two types of reflector on it. The reflectors are for guidance in the dark, heavy snowfall or indeed a flood as per recent events in May of this year and to help you identify where the edge of the road is.
I haven’t looked it up, but to my untrained eye, the vertical reflector is basically telling me to follow the road (think of it as an arrow) and that you are on a correct side of the road, the two dot reflectors on the other side, are those representing a pair of headlight of the oncoming traffic.
Growing up in Lithuania, this type of road marker has arrived in the country a few years after the fall of USSR. They did however, only have a white, amber and red reflectors on them.
*73km equites to 73000m, or 7300 posts a side meaning 14600 post in total on just on that stretch of the 73km long road.
**It is not to different to motorway markers here in the UK. Which I doubt even half the population*** know what they are, where to locate them or indeed how to use them. What does A and B mean for example on the M25. Meaning that when they are calling through for help from the side of the motorway, and asked for their location they just shout down the phone that they are somwhere near an old oak tree and a church in the distance
***unless one is a professional driver such as trucker, emergency vehicle driver etc.
Road signs are there for a reason, it is just a shame that majority of people haven’t a clue what they mean or simply ignorant and not interested to know. Knowing what is happening on TikTok or some such entertainment outlet is way more important unfortunately.